The Quiet Piston-Driven .300 Blackout Pistol: An Obtainable Unicorn

I must admit, I’ve had two very prevalent fascinations over the past year or so: Sound suppressors and running a .300 Blackout AR-15 pistol with an aftermarket piston system. The first fascination compliments the second fascination, so the two marry especially well. However, the second fascination is the problematic player in the relationship. I can make (and have made) a piston driven AR-15 with an 8” .300 Blackout barrel cycle supersonic ammunition just fine, but anyone who knows anything about shooting .300 Blackout knows that the real magic happens when subsonic ammunition meets Mr. Sound Suppressor. The problem lies in lower pressure causing cyclic unreliability.

The question I get asked the most when telling people about my dilemma is, “Why are you even bothering with a piston system? My direct impingement gun cycles supersonics and subsonics just fine, even without a suppressor *rabble rabble rabble*.” My answer is always: Why not? I have been tainted by the luxury of piston driven ARs, and I will not let my borderline-unrealistic dreams be coaxed astray by your ‘facts’. Here are some of my own ‘facts’:

  1. Piston ARs are much simpler to clean. Anyone who has shot a direct impingement AR-15 knows how caked the bolt and carrier components can get caked with carbon fouling, and cleaning off said fouling is a time consuming task all in itself (which cuts into my precious youtube cat video time). Ever since I switched to a piston driven AR, I’ve not had to scrub, scrape, or soak any part of my AR. Cleaning with a piston takes roughly 10 minutes, TOPS (just the barrel and a little wipe down in the upper).
  2. There is MUCH less gas in the face when shooting suppressed.
  3. I’ve been to the future; everyone uses pistons… And Apple comes out with the iCar. It drives itself and has room for four people and over 100,000 songs. I think it has issues though: it gets bogged down with apps and it won’t let you delete the songs you don’t want to hear anymore.

So, in my journey to establish a piston driven .300 Blackout Pistol that cycles reliably, I’ve discovered four very specific and sensitive variables that impact the success of these little nubblet firearms: The barrel, the piston system and bolt carrier, the recoil buffer spring, and the load recipe. Here is the setup I’ve chosen to run:

  • Barrel: McGowen 9” Stainless steel pistol length gas system barrel (.300 Blackout, of course) with a 1 in 8” rate of twist.
  • Piston System: Adams Arms XLP Pistol length kit with low mass bolt carrier (important)
  • Recoil Buffer Spring: Standard (cut)
  • Load Recipe: 220gr-230gr projectile, 11.0gr-11.2gr Accurate 1680

The functioning product

The Barrel

The first and most important thing to know about running a .300 Blackout with a piston system is that it should never have a barrel longer than 16 inches and should always have a pistol length gas system (regardless of barrel length); it’s going to need as much gas as it can get. Second, the gas port size will impact functionality and reliability. Many .300 blackout barrels will come with a gas port diameter of .075”-.090”. If you are running a 12”-16” barrel, that diameter range will work just fine. If you are running under 12”, you might find the gun running quite under gassed. I found that for short barrels, a piston system will need a gas port of .10”-.12”. That might seem large, but again, if you want reliability with subsonic ammunition, it will be necessary to let in as much gas as safely possible. The McGowen came stock with a .090″ gas port, and it had issues. So, I drilled it out to about .11” and it is now cycling everything just fine with the regulator on full-blast. If you are worried about overgassing, don’t be. The Adams Arms XLP gas block can be regulated with five different settings, so if it winds up over gassed, you can always turn it down.

The Piston System

There are many AR-15 piston systems on the market today, but I chose to go with the Adams Arms XLP kit. Why? Because it was the only one I could find in a Pistol length setup that had more than just three general regulator settings (and like my gas to be custom, dawg). Now, there is one problem with the XLP kit direct from the factory: The piston return spring is much too strong to reliably cycle our little subsonic nibblers. After a brief bout of GoogleFu, I found an alternate, lighter weight spring that can be obtained at ACE hardware; item number 540425 (you’re welcome).

As of 12/28/2015, a commenter named Bert Payne informed me that there is a better option. He states that the Adams Arms Stock spring is .038″ wire, .306″ outer diameter, 1.5″ length, with spring weights of 2.7lbs installed and 8.8lbs compressed. A Hillman “General Purpose” spring (ACE item number 540371) would be the best option, with a wire diameter of .032″, an outer diameter of .281″, and spring weights of less than 1.15lbs installed and 3.2lbs compressed. Best of all, it’s slightly shorter than the 540425 spring and doesn’t need to be cut. Thanks Bert!

The low mass bolt carrier that comes as an option with the Adams Arms kit is pretty damn excellent. It looks quite dapper with its undulated cuts, and the lower mass seems to help with cycling. Whether or not the lower mass has a gigantic impact on functionality is up for speculation. Regardless, it’s a good option, but it will increase the price of the XLP kit by quite a bit.

The Recoil Buffer Spring

Cut buffer spring below an uncut buffer spring

Now, this portion may cause a little uproar on the interwebs. I utilized a standard weight recoil buffer and spring, but I cut the recoil buffer spring down to about 9.5″ to lower its overall spring weight. There is still plenty of weight in the spring, but it is considerably lighter. This actually made a HUGE difference in cycling subsonics. You can probably accomplish this if you buy a lighter weight buffer spring, but who actually buys anything in this day and age?

The Recipe

This is going to be the keystone of the entire setup. You can have a .300 blackout that can cycle factory supersonic ammunition, but if it’s not quiet, what’s the point of having it? Well, companies such as Remington, Nosler, HPR, and Sig Sauer have factory loaded subsonic .300 Blackout ammunition available, but not one of those manufacturers loads have successfully cycled my piston gun. One could extrapolate that the factory subsonics are designed for direct impingement guns sporting 16″ barrels (and I don’t humor any of that garbage). Thus, I decided to control my own destiny and chose to roll my own custom-tailored ammunition to accommodate my particular needs.

After months of trying out several different bullet and powder combinations, I’ve concluded that the ideal load for my gun is a 220gr to 230gr projectile sitting atop 11.2gr of magic pixie dust: Accurate 1680. According to Hornady’s load manual, 11.0gr of Accurate 1680 behind a 220gr projectile will produce a little under 1100 fps out of a 16” barrel. Since Wikipedia told me that 1125 fps is the speed of sound at sea level, in dry air, with an ambient temperature of 70° F (and since it’s on the internet, it has to be true), I chose this as my starting load with the belief that my shorter barrel would equate to lower velocities.

My initial tests began with some beautifully powder-coated 230gr cast boolits made by a buddy of mine who is into the casting scene. Without a sound suppressor, this load was definitely subsonic. They had a fairly mild pop and produced no supersonic crack down range; albeit, they did not cycle the action. Once I put the sound suppressor on, the round fully cycled the action (HURRAY!), but it definitely produced a supersonic crack down range (NO ME GUSTA). How could there be such a change in velocity change between unsuppressed and suppressed? Well, Mr. Chronograph tells me that these rounds are pushing about 1085 fps on average WITH a suppressor, so why are they producing a crack? It turns out that has to do with some fairly interesting factors in aerodynamics.

(WARNING: I AM NOT A SCIENTIST OR PHYSICIST – AFTER FURTHER RESEARCH, ALL OF THE FOLLOWING WAS PROVEN TO BE UTTER MALARKEY. IT HAS BEEN LEFT IN TO SHOW HOW HILARIOUSLY DUMB IT WAS; THIS WILL BE ADDRESSED IN NEXT BLOG POST) Since the bullet is traveling near, but not quite at the speed of sound, it finds itself in a state of transonic flight. That is, the bullet might be traveling below the speed of sound, but the air travelling around the bullet (being pushed out of the way while the projectile moves through it) may be moving faster than the speed of sound; this will still produce a supersonic crack. So, my tiny brain logic dictates that if a projectile is very aerodynamic, the surrounding air will be able to get around the projectile at a lower velocity than a projectile that isn’t very aerodynamic. Allow me to use pictures shamelessly stolen from to illustrate my ‘logic’:

more aerodynamic less aerodynamic

As you can see in the pictures above, the more aerodynamic projectile (on the left) generates a much shallower angle in its bow shock during supersonic flight than the less aerodynamic projectile in supersonic flight (on the right). Because of that, I can conclude (in my completely unprofessional and uneducated opinion) that a shallower angle in bow shock = lower velocity of air traveling around the projectile. So, the ideal projectile for this situation will be a heavy-for-caliber projectile with a long ogive and high G7 ballistic coefficient. I’d say a .308 diameter, 210gr VLD Hunter or 230gr Hybrid Target bullet from Berger would do nicely (I will be testing the 210gr bullets). Their price is a little steep, so after taking a small loan out and selling a kidney, I procured a box of 210gr Berger VLD Hunter bullets to test out this hypothesis.

I’ve tried a few different powders that many internet warriors have claimed will produce a cycling subsonic round in an AR-15 pistol, but only one powder takes the piston cake: Accurate 1680. It’s the slowest burning out of all of the powder choices suggested and it works beautifully in subsonic .300 Blackout. The load recipe I’ve found that works best is:

Bullet: 210 gr Berger VLD or Hybrid Target

That moment when the bullet is longer than the case.

Powder: Accurate 1680 11.0gr-11.2gr (this will cycle)

Primer:  CCI 400 Small Rifle (really, any small rifle primer will work)

Case: Assorted converted .223 Brass (not 5.56 – don’t want to turn necks)

Deity: Christian Denomination God (to pray to so they work; truthfully, your choice of deity is important here)

The morning you decide to shoot, make sure you follow these steps exactly as laid out: First, wake up and look at your cell phone. Put cell phone down, get up, and eat breakfast in the shower (great way to save time!). Dry off, put clothes on WHILE brushing your teeth, deoderize, feed the cat, grab your range gear (not the cat), gun, and AMMUNITION (ammo is most important) and go to the range. Hopefully it will be a quiet day at the range so you can actually hear for supersonic cracks… and shoot at paper, not steel (everyone shoots subsonics at steel on youtube… don’t be a tool). Finally, you will hopefully get this to happen (sorry, I have no camera person and they were about to call a cease fire):

Conclusion on Berger Load: Chronograph clocked them at an average of 1080 fps, and although there was an echo down range, I don’t think it was a super sonic crack… It was hard to tell at the range, and just as difficult to discern from the video. I think the hypothesis needs further exploration… Regardless, the gun is cycling, bolt locks back on empty, and it’s fairly quiet. I’d say ‘Mission Accomplished’…? And just in case you come to an existential crisis in deciding to do this to your gun, just know that supersonic rounds still work just fine in this setup without any modification; just don’t shoot them through your form 1 MagLight suppressor (it’s a good way to turn it into a $290 projectile, though).

If you’ve had different experiences or have had success making a piston driven AR pistol in .300 Blackout work, please feel free to share your comments below. The more information we have out there, the less likely we’ll need to pay for a Sig Sauer MCX or PWS MK109 to get our .300 Blackout piston fix.


55 thoughts on “The Quiet Piston-Driven .300 Blackout Pistol: An Obtainable Unicorn

      1. Bert here – Subsonic bullets are not cheap (although Berry’s plated 220gn might be a game changer – for plinking). The most popular bullets for .308 are 155gn and 168gn, and quality bullets can be found for under 20c. The trick for light subsonics is to seat them low (like Vmax low). This keeps the powder from sloshing around, and keeps your SD in line. I’ve been using 155gn Amax and 168 Tipped Match Kings (XMas colors). Since they use half the powder, they come out to just 26c a round!


  1. Nice article. I went with a slightly different approach that led me to the same conclusion as you and ultimately to your article. I purchased an AA 9.5″ upper in 300 BO direct from AA before they discontinued them. Adams Arms knew they had a problem with these uppers but sold them anyway. My first experience with an Adams Arms upper has not been pleasant and certainly not worth the $1000 I paid for it. The upper would not cycle anything supersonic or subsonic. I ended up replacing the piston spring just as you did. It got me close to reliability on the subsonics with a suppressor. I have tried to find the spring you used #540425 and can find no such item at Ace Hardware so please post some additional info so we can find the same exact spring. They have a lot of springs but without knowing the length, diameter and poundage of the one you used, your info is not much help. The AA barrel gas port was .998 inches. I drilled it out to .125. I am using a buffer with all the weights removed and have cut my buffer spring multiple times. One of the biggest issues is that the AA XLP piston plug becomes frozen in place after firing a few shots and it will not adjust at all. You certainly cannot adjust it after putting a rail on the gun, because there is no way to reach the plug and apply enough force to make it turn. It takes a wrench and a lot of force to convince the gas plug to turn. I also had to replace my AA barrel because it was a 1 x 9.5 twist rate and the heavy 220 gr. subsonic bullets would tumble. If I had it to do over again, I would have purchased a PWS upper in 300 blk and been done with it. My rifle is still not 100% reliable and it will only cycle subs if there is a can mounted on it. Adams Arms sold me a pig in poke and it will be the last time they get my money.


    1. Hi Stuart,
      I’ll try to answer all of your questions as best I can.

      1. I think I have a spare spring at home and I might still have the packaging. I’ll take some measurements and try to see if I can send you some additional information about it.

      2. Are you hand loading or using factory ammo? I have had no luck with factory ammo, so I only load my own. In addition, my gun can’t run subsonic ammunition without a sound suppressor. I can run supers without the can just fine, but subs need a can.

      3. “One of the biggest issues is that the AA XLP piston plug becomes frozen in place after firing a few shots and it will not adjust at all. “ THIS. VERY THIS. I have had the exact same experience with the XLP gas block. While trying to calibrate the gas timing, I had to do shoot without the rail so I could tune the gas block. This was actually a waste of time for me, because it worked best at the 100% gas setting. I left the gas on full blast and it has functioned 100% ever since.

      4. Do you have the low mass bolt carrier? I have the low mass carrier and it might be the differentiating factor.

      5. I agree with your choice to replace the barrel. 1 in 9.5″ is too slow to stabilize them 220gr pills. Mine is a 1 in 8″ and it works beautifully with supers and subs.

      6. Are you running your gun with lots of lube? I run my gun super sloppy and it made the difference between cycling and not cycling. Also, I have a POF roller cam pin. You might want to consider getting one. It was one of the best investments I ever made.

      7. As for running reliably without a sound suppressor attached, I can’t run subs without my can either. I guess I justify this because I see no point in shooting subsonic ammunition unless I have a can on the gun. Just expect the gun to have cyclic issues when testing for stability without the can. My gun is able to run supers with or without the can just fine.

      I wanted the PWS upper in 300 blackout also… It’s just so freaking expensive. Albeit, my upper didn’t start at $1000, so I can see your frustration.


  2. Yes, I am rolling my own ammo. Factory ammo will almost always not cycle and ends up going supersonic. I am running the low mass carrier with lots of lube. (this was supposed to be included with the upper when I bought it, it took me a few months to realize they sent me the wrong carrier and that is why it would not cycle the supersonics). My new barrel is 1 in 8 twist. If I shoot supersonics, it is now over gassed and thus the reason I need to adjust the gas plug. My guess is that the plug is becoming very fouled after just a few rounds and results in not being able to adjust it. What is the point in having an adjustable gas block if it cannot be adjusted? Also, I had a typo above, my original gas port in the barrel was .098. Thanks for the tip on the roller cam pin. I will give that a try.


    1. Stuart,
      I couldn’t find the packaging for the spring, but I did find the forum post where I got the idea. In the post you will find a picture of the spring and how short it was cut. I started working on my build about 3 months after this person did, so there is a chance they may not carry the spring anymore. He also mentioned that the reliability improved considerably once the piston spring was replaced.


  3. The real question that I have here is …. with this work/cycle properly using off the shelf subsonic ammo like prograde, hornady or remington?


  4. Here is my build, running a 7.5″ barrel and Adams Arms XLP with Low Mass Nib Carrier and MVB Arc Stock – I did a lot of searching and had to replace all weights in the buffer with 1/2″ Delrin Rod since the stock has proprietary springs. Cycles perfect with supers unsupressed and perfect with subs suppressed by a Saker 762

    BTW have tried a ton of subsonic and my favorite are the AE300BLKSUP2 rounds so far, gemtech cycle well too, very consistent, hope this helps! It’s been super reliable after reducing the mass behind the carrier without bolt bounce, if you can change your springs maybe try a Sprinco White Spring

    Big thanks to Adams Arms again for their great customer service!


    1. Well… Damn… So what was the weight in the buffer? Delrin is what (plastic)? What barrel and original/drilled gas port? Did you try to modify the springs at all? I was going to try flatwire springs.


  5. There are several springs at ACE that fit the bill. They are Hillman “General Purpose” springs, with specs on their website. The 540425 has less coils once cut – not the best choice (binding). The 540416 is better (same number of coils cut as the AA spring). Best is the 540371 which is slightly shorter but doesn’t have to be cut at all. For tech nerds: The AA spring is .038 wire/.306 OD/1.5 len (2.7lbs installed/8.8lbs compressed). The Hillman/ACE springs above are .026, .028, .032 wire respectively, 9/32 OD. They reduce the power to less than 1.15lbs installed/3.2lbs compressed. GoogleFu..!!


  6. My local Ace Hardware had places (bins) for some of the springs listed by others but never had them in stock. I lucked out and found a better assortment of springs at a local True Value hardware store. The spring I settled on was in a small clear bag with green and white labeling that said “Handi-Pack”, Part # 88247, 5/16 x 1 11/16, compression spring. UPC #738287882476. I did not cut or modify the spring.
    This spring was less than $1 and has solved all my problems. I ended up having to go back to a heavy buffer to slow the speed down. My rifle now cycles everything I have put through it. It is even cycling some subsonic handloads without the suppressor. Suppressed, it has cycled everything I have put through it so far, approximately 1000 rounds of all different brands and handloads with perfect reliability.
    So here is the final recipe that has worked for me:

    *Adams Arms XLP pistol length piston block with the drive rod spring replaced with the spring above.
    *Tubbs flat wire buffer tube spring with a few coils cut off (can’t remember how many)
    *9.5 inch 1 in 7 twist barrel with the gas port drilled to .125
    *POF Roller cam pin ( I was skeptical -but this made a huge difference and I have purchased them for every AR I own now)
    *AA low mass bolt carrier
    *Now run the bolt really wet with Superlube lubricant
    *Replaced my buffer tube to one that has no rough machining inside (super smooth tube)
    *Running superlube in the buffer tube with the Tubb’s spring
    *Spikes ST-T2 buffer
    *Cut a notch in my Samson EVO Rail that allows me to reach the XLP gas adjustment lever
    *Running the AA XLP piston internals with Superlube (I can now move it after shooting)
    *Cut the M4 feed ramps a little deeper and polished them (video on youtube shows how to do this)

    The end result is that the bolt is running super smooth. Like it is on ball bearings. Now . . .If you offered to trade me even a PWS MK109, I would have to decline. Love it!

    Liked by 1 person

    1. Interesting (love that POF but $$$)! I assume you are running subsonic rounds with a suppressor. What ammo? Have you been able to cycle the “notorious” Hornady 208 grain? Some reliability upgrades – the bolt catch is loose by default, when carrier drags over it, it leans back away from magazine follower – 300 Blackout ammo is heavy (10 rnds of 150gr/220gr makes mag 1-2 oz front heavy – full 30 rnd mag can be 11+ oz front heavy!) so it leans forward. The follower only has 1/16 inch contact with bolt catch so bolt lean back/mag lean forward… shims are available – this causes many of the bolt back/round feed intermittent issues.


  7. I’m working through some of these issues myself. I’m reworking 300 BLK build with an AA XLP on a 8.2″ CMMG barrel. I went to the piston as the DI system I had ran really well, but it was absolutely filthy. After ~ 200 rounds, you needed a hammer to open the bolt (no exaggeration). Anyway, with subsonics, I’m very close to getting the bolt to lock open on last round and it will feed a new round occasionally. Really all I’ve done so far is open the gas port up to ~0.105. I’m hoping the light guide rod spring will get me there…..just have to find one. My big question is are you still able to run full velocity rounds without damaging the gun with the light weight springs? I’ve also looked at swapping the buffer spring to a JP rifle silent capture recoil spring as you can get them in a kit different spring weights. I’ve had great luck with them on other DI builds with suppressors and adjustable gas systems. Does anyone have any experience with them for these piston builds?


    1. Mine runs supers just fine without any damage to the gun. The lighter weight piston spring will make a very large difference in functionality, so good luck on the hunt. I’ve never used the JP captured spring system, so I can’t give any advice there.


  8. Revelation! One of the great things about 300 Blackout is the scrutiny of every convention. Here is another (especial for AA pistons). Reverse the order of the buffer weights. Conventional thinking is that the heaviest weight should go toward the back (seemingly logical reasons). However putting the heaviest weight forward (at the bottom/closest to carrier/opposite bumper) changes some things. It initiates the inertial delay (the purpose of the buffer) immediately, rather than moving the carrier 1/8″ to stack the spacers and weights for affect. The AA piston’s challenge is that it is designed to exhaust excess gas – with the blackout we need ALL of the gas (this is why the AA needs 15-20% more gas than DI). If the piston moves 1/8″ (stopping to pull reluctant case) you shorten the stroke (only 1/2 in the first place), and open the cup. If the lugs turn, but the case still doesn’t come out you lose another 1/8″ stroke (the carrier stops again), and the cup opens more. Every time the carrier stops you have to overcome inertia again – up to three times! Immediate delay of the buffer keeps the piston cup closed, preserving gas, time for the case to shrink, before it begins to move. If you use a 3oz+ buffer, try a tungsten weight with aluminum weights (KAK sells individual buffer weights). Or go lighter with steel/aluminum/aluminum (2.2oz). (or at home depot – 1/2 steel nipple – .25 oz each or an even lighter spacer).

    Liked by 1 person

    1. Sorry. The port size is 15-20% “wider”. The actual required gas increase (Adams Arms) is 40+%. Ex. from say .105 to .125 is a 15% wider gas port, but is actually 40% more gas (π × r2).


  9. I tried to get my 300 blk to run an adams arms piston set up about 6 months ago, and had all but given up on getting it to run. After reading through all this I have some different weight springs to try in the front, I already have a JP Captive buffer will all weights and spring weights for tuning in the rear, Hopefully in the next month i’ll come back with good news.


  10. Consider this… The AK-47 uses a piston (so many rifles do – in fact DI is in the minority). The starting gas port size is .125 in. (1/8″) – that is with a considerably larger case for more powder. For those nervous about opening the gas port, don’t be.


  11. Thank you for posting this article, it was the first to come up when I was searching for “building .300 blk piston upper” so muchas gracias. I am seeking to build a .300 blk upper for my LWRC pdw. A piston system is required as is respect for the shortened buffer tube of the PDW. I presume that I can cross utilize the BCG from the LWRC upper (it already is set up for the shortened bugger tube and had the Delrin buffer at the rear of the BCG? This would be a first-time build for me, so I will be getting assistance from a friend who’s build a dozen or so DI guns and has the specialized tools. Any advice to the noob regarding this setup? Many thanks, J


    1. Sounds like a fun project. Be aware that all companies have different dimensions for their bolt carrier (in relation to piston length and so forth). I would suggest getting a matching bolt carrier for the piston system you are using. As for the Delrin buffer, it’s not a bad idea, but it will most likely give way due to abuse when it impacts the back of the buffer tube. I’d say try to mill one out of Aluminum if you can with an indexing nipple to go into your bolt carrier (so it doesn’t lose center), then but a little rubber bumper on the rear portion (gotta protect yo’ shorty buffer tube). You may have to tinker with buffer spring weights, but it’s obtainable. Good luck on your build! Also, are you planning on using one of those H&K-style wire collapsible stocks? If so, avoid it at all costs. They are awful. They look cool, but are just plain awful to use.


  12. Im really happy to have come across this post. I’m planning on building a gas piston 300 Blackout upper and this is the most comprehensive account of that process I have come across. DI is fine for some…. But like you, I’ve already been tainted by the luxury of a piston driven AR. No chance that I’ll be reverting back to DI for any reason. Thanks for the great post. If I have anything to contribute at any point, I’ll be sure to leave additional comments.


  13. My two cents. Just wanted to thank you all for your insight. When I decided to build a piston 300 pistol I didnt realize id run into hick ups. I normally do my research before I start a build. That being said I wanted a pistol small enough and light enough to carry in my backpack. I have no intention of running subsonic or suppressed.I went with the AA piston with low mass carrier and Sampson rail. In order to shorten it enough to fit my backpack I installed a Law Tactical folding stock adapter. Upon test firing not a single round would extract. I tore it apart and checked gas block alignment and tried again…same thing. Then I found this page and started reading. The folding stock adapter added about two inches in length to the bolt carrier and of course more weight. I removed the weights from the buffer and again test fired. This time all rounds ejected.But it would not feed the next round. For $15 I installed a softer buffer spring. Now it runs flawlessly.


    1. It is truly amazing how much a buffer spring impacts the function of the gun. Glad to hear that it’s working now! As strange as it sounds, we who have piston operated pistol ARs in 300 Blackout are still a small group of people. Welcome to the group!


  14. I too am really curious about the JP Silent Captured Spring idea. After visiting their website just recently, it looks like they’ve partnered up with Maxim Defence for a tube and stock project and it looks amazing! Especially for this type of application! I don’t have the tools or know how to do a project like this but hope to work with someone in the near future that could help accomplish this. I’m curious some of your guys’ thoughts and opinions on doing something that may be considered extreme: I’d like to cut my barrel down to 5″…I’ve seen a YouTube video of this posted by High Caliber Conversions LLC and it was cycling…but have not been able to find much more info on what kind of mods need to take place for that.

    Love the article, love everyone’s input and excited to see updates

    Thank you


    1. The maxim defense stock does not work for this. The spring is a different wind than the rest of their springs. I tried both the 308 and 556 springs with it. And once you got it loose enough to cycle, it would not close the bolt. It’s a great stock otherwise.


  15. What would drilling the gas port on the barrel make any difference if the hole on the gas block setting is smaller, that is what regulates the amount of gas, are you guys drilling that out too?


    1. It might be hard to believe, but in the Adams Arms piston kits, the port size in the gas block is enormous… much bigger than the .11″ I drilled my barrel’s gas port out to. I keep my gas setting at 100% to ensure as much gas gets in that piston as possible. Drilling it out made a huge impact. You shouldn’t need to drill out the port in the gas block.


  16. Crap, I was really hoping there was a way. I see they have different springs on their site, but I guess even using a slightly weaker spring results in cycling but not chambering, as you say…thank you for your input.


  17. Question about gas pressures: I built a setup almost exactly like yours, 8″ barrel, YHM Suppressor, AA piston, .125″ gas port, light buffer spring, Ace hardware piston spring, load recipe, only diff I can see is the barrel brand. I’ve been working on this setup for 2 years & I still don’t come near cycling Subs… I’m thinking of trying a 10″ or 12″ barrel for more dwell time. But it doesn’t seem like the pressure would increase with a longer barrel because it would increase the velocity of the bullet, so I’d load less powder & obviously less pressure. Does this sound correct that a longer barrel would not help my issue?


    1. You are 100% correct. A longer barrel = greater distance from gas port to end of muzzle = greater dwell time. I have a 9″ barrel, which does add dwell time, and that could be the difference between cycling and not cycling. HOWEVER, before I got my McGowen 9″ barrel, I had a 7.5″ barrel I bought from a place that doesn’t exist anymore, and I had no problems with cycling at all (other than it was so short that my suppressor pushed up against the release button on the piston system, which made me have to get a long thread adapter to use the suppressor).

      I’m going to assume that you’ve already checked for gas port alignment, excessive friction on the op rod (like minute misalignment with the upper or contact with handguards), and full piston travel (sometimes the replacement piston spring can impede the travel of the piston. Verify that the shoulder on the op rod is making full contact with the bushing when all the way back. If the piston is short stroking, it will not cycle at all. I know all about this because I’ve had this happen). If so then troubleshoot with this list prior to losing hope on this barrel:

      1. Friction can slow your gun down enough to prevent proper cycling. How lubed are you shooting your gun? I like to run my gun dry, but in the beginning, I had to run it SUUUUUPER sloppy with lube. I used Outers Gun Oil (the stuff in the pump bottle, not the aerosol) because it doesn’t evaporate and it is super slick. Also, take your buffer spring out and give it a healthy coat of grease. Molybdenum grease works great. Tetra Gun Grease also works, but I like the moly grease better. Not only will it make the gun cycle easier, it will make it much smoother and quieter. I also use a POF Roller Cam Pin, which cuts down friction considerably. I’ve never tried to shoot it with a regular cam pin, so I don’t know if it really makes a difference.

      2. (silly question) Have you removed the gas rings from the bolt? The gas rings add quite a bit of friction and are obviously unnecessary in a piston gun. If you did remove them, then ignore this question.

      3. Are you using the Adams Arms XLP kit with the low mass bolt carrier? I assume yes. If not, the weight of the bolt carrier might be slowing you down.

      4. What weight buffer are you using? Try reducing/removing the weights from your buffer to further reduce the weight. Just know that if you remove the buffer weights, don’t try to bumpfire or shoot full auto. Those weights are designed to help mitigate carrier bounce, which really only applies to full auto/bumpfiring.

      If all of those things fail, you might be doomed to buy a longer barrel. Sorry bro.


  18. Just finished my build. Mimicked Stuart’s build but with an eight inch barrel. Cycles sub and super rounds. I’m setting up my tooling to roll my own ammo and have a question.

    Are you putting a crimp on your case?



  19. Now, almost a year after your initial post… The SIG MCX 300 shortie is still a unicorn. I’m guessing that’s for a reason? LWRC, who ARE piston-driven, released their 300 blackout shortie – but in DI. Doh! LWRC went DI for their 300???? Man, that threw me off alight. That’s right about when I started to realize that maybe — for me — this was a bit like chasing a ghost. The MK 109 is known to be under-gassed, but is still the only instance that I can find (on the interwebs) of an actual factory-built, piston-driven 300 shortie that has a ‘good’ rep.

    I too shared your vision. I actually referred to it as my ‘holy grail gun’; a piston-driven (for me, factory-built) 300 AAC shortie. And when the MCX 300 pistol was announced and touted and glamorized in numerous Shot Show videos, I got a little woody, thinking – could this be it? Well, I think the Big Guy might have taken a little offense to my holy grail reference… how many years later is it and the only reference I can find is one dude on a Sig Forum who managed to get an early release (before they shut it down) and had nothing but problems.

    But, things aren’t all that bad, because right around that same time, I managed to score a limited edition AAC MPW 9 pistol… Yes, it’s DI. I’ve since embraced it. I’ve accepted that as my punishment for the whole ‘holy grail’ piston delusion I once had. The MPW is a gem though. Sure, it might be gassier than my Grandmother after a bowl of baked beans, but she shoots one damn ragged hole. I almost can’t believe how accurate this gun is with the right ammo. She cycles almost everything unsuppressed, all the way up to 208 gr AMAX at a measly 1050 fps (although just barely). Shoots the factory 220’s with no issue even unsuppressed, and loves 110 supers of course. 110’s without the can is really my preferred defensive loading/set up anyway (Barnes TacTX or Hornady VMAX). With the SDN-6 on, yeah… she’s gassy alright. But I just managed to score one of the new AXTS ‘Freedom Bones’ (what a cheesy name) and man, that thing makes a world of difference. Now most of the extra gas goes out the chamber. The only, only slight draw back is that it does get a little dirty, yeah. Not a piston. She shits where she eats. I get it. I have to clean her up after a trip to the range, but it’s really not all that bad. And in trade off, I get the supreme accuracy, simplicity and reliability of a DI gun. The MPW itself is all top shelf, KAC rail, Geissele trigger, a nibx MPI/HPT BCG, low-pro pinned block on a friggin cherry barrel with a 1:8 twist and an AAC FH (which I now keep the new AAC Blast Out over when not suppressed, to keep what little blast there is all down range).

    As you can tell, my path was quite different, my equation quite different – as I don’t reload (don’t have the place yet), and I do not yet have the wear-with-all (gunsmithing skills, bench, etc) to really build my own as you have. So for me, it was really OEM or try to do something that’s outside of my practical experience and skill set. I saw the AA’s when they first came out and almost jumped on it, until I saw that you can’t adjust the gas setting without taking the damn rail off. That pissed me off to no avail. As soon as I ‘gave up the ghost’ of a piston-driven 300 AAC (Factory built) shortie, it all came together for me and it’s been a blast ever since. Until I can get a place with a bench and start learning the art of reloading, my quest now is to find the best cheap reman range ammo and have been experimenting with all of the big names, and some local shops too. In the meantime, every once in a while I’ll scan the interwebs for traces of the MCX 300 shortie and get a little laugh that it is still a fictional item. Glad I didn’t wait around for it. Maybe some day that will change. Maybe some day I’ll get a bench and some actual skills to do a build from scratch myself. Until then, it’s all MPW for me, and loving every minute of it (until of course I have to pay for more ammo – ouch!) :)

    This article was awesome by the way. Hugely informative. Thanks.


  20. Hey. Great article and great comments.

    Just a heads up- when you cut a spring to make it shorter, you’re actually making the stiffness of the spring greater- it’s stiffer. When you install it, it has less preload, so maybe less initial force just sitting there- so it would (maybe) take less force to get it started moving- but for sure it’s stiffer. One of the things in the equation to calculate spring rate is the number of active coils in the spring. When you cut it, you are decreasing the number of active coils- increasing the spring rate… (Ping me if that doesn’t make sense.)


    1. Yes, but by reducing the pre-load on the spring, the overall compression is reduced when the buffer reaches the back of the buffer tube. Don’t get me wrong; by cutting coils off, you are changing the properties of the spring substantially. We never made it to spring rates in my entry level physics class in college, so I’m not going to say I actually calculated the length to get it right. It just so happens that the amount I cut off works perfectly to keep proper function.


  21. Lots of information here. My story in dealing with the AA piston 300 is quite similar to everyone else here. Thanks for the article. I’m happy to report that my 9″ AA upper functions 100% reliably. Mine is roughly a 2 MOA gun. Can anyone give me some feedback on accuracy?


    1. I’m getting about 1.5 MOA out of mine. I’d like to believe it’s probably due to the fact that I am a poor shot. I don’t really use the gun for much else than up close shooting and potentially pig hunting down south, so I suppose that margin of accuracy suits my needs.


      1. It’s upsetting when I have to strenuously focus on the fundamentals just to get a 2 MOA group. I’ve just accepted it as a short-range (less than 200 yards/meters) weapon system. Owning a blackout is going to get me into reloading. I can’t afford to buy factory ammunition all the time. I’m assuming reloads would be significantly more accurate.


  22. I had a hard time finding 300blk factory loads for much less than a dollar a round, until my buddy (who used to work at Adams Arms) turned me on to
    Excellent pricing, best I’ve seen anywhere. Has anyone tried the vltor A5 system to remedy cycling issues? I hear nothing but wonderful things about the A5 system. Any input would be greatly appreciated.


  23. Anyone try superlative arms piston kit yet? I bought one because it would fit under my giessele rail. I started my build and am waiting on the piston to come in to complete it. I’m so great full for this page because it has amazing info on it. Thank you guys! I’ll post my experience once I get a few rounds down range


      1. I actually know someone who knows the owner of superlative… I will pick his brain and get back to you guys. I also know that Adams Arms is working on a much improved XLP piston kit due in August that apparently works much better and has fewer moving parts and is actually adjustable by hand after firing more than three rounds. :) According to one of my good friends that works there, it’s supposedly the bees knees and will cycle just about anything…The second it comes out it’s going on my 7.5″ 300 black AR, so I will give you a full report after testing.


      2. It runs supers unsuppressed with my jp silent capture with the lightest spring, won’t run subs unsuppressed, I did however purchase the nemo spring to so how that goes. I’m also getting ready to purchase a suppressor soon. I’ll keep you guys updated


  24. The ammo is barely going supersonic with the can because the can increase velocity. That is why you are having the supersonic crack.


  25. I’m “in the industry” and have talked with Tony Russo (the owner of Superlative Arms. (NEVER EVER MENTION SYRAC when you speak with them, it’s a very bad history) They are working on a dedicated 300 Blackout pistol length piston kit. That’s all I can say right now. This info is a week old. His current pistol kit apparently will not work although I’m trying to build one right now to make it work based on his suggestions. My biggest problem is that his current piston kit will not fit under a 2A Armament hand guard. I’ll have to do some machining to the hand guard to make it fit. It’s also advisable to get a JP Silent Captured buffer system and their spring kit.
    I should have some more info shortly on my build. Based on some info from Tony I think I can get it running.


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