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 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

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)

Me: 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 aerospaceweb.org to illustrate my ‘logic’:

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…

UPDATE: 3/12/19

Except all of that that is completely wrong. The reality of this phenomenon is actually the complete opposite. The more ‘V’-like the bow shock (which is the physical manifestation of the sonic boom), the faster the projectile is moving. Most importantly, you can actually see that there are multiple lines coming off of the projectile. That’s because air moving over every little ridge and scratch is also breaking the sound barrier. I find that CRAZY, because it means that the projectile is creating a sonic boom not only at the nose of the projectile, but along multiple locations along the corpus of the projectile. NEAT.

You know, I can try to explain this, but a video by Smarter Every Day already exists that does a phenomenal job explaining this phenomenon. It’s starts at about 2:35 mark in the video. Ignore the in-video advertising this guy does… the rest of the video content is actually fascinating.

Back to the concept at hand, since every single little ridge and ledge on the projectile will cause the air moving over those ridges to make a miniature supersonic crack, the ideal projectile for this situation will be a heavy-for-caliber projectile with a long ogive and super slippery 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

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.