Piston Driven KS47 – Chapter 2: The Build

Chapter 2: The Build

What might one need to build a monstrosity like this?

In order to build an AK-mag-fed AR, you will need, at a minimum, three skeletal items:

  1. An AR lower that accepts AK-47 magazines (PSA KS47 Lower – $149.99 on sale, $199.99 normally)
  2. An AR upper with proper clearance for AK-47 magazines (PSA KS47 Upper – $139.99)
  3. A modified bolt carrier group with proper clearance to work with AK-47 Magazines (PSA KS47 Hybrid bolt carrier group – $199.99)

All other parts are just regular old AR-15 parts. Grab any 7.62x39mm AR barrel from any barrel manufacturer, a standard 7.62×39 bolt, and you are in business. I suggest you buy a quality bolt, at least. (Bolts go from $60 up to $200 – you usually get what you pay for)

At this point, I’m breaking from the norm by choosing to use a piston system. Doing so will require modification to the bolt carrier – more on that later. I happened to come into ownership of the upper and lower receiver, so they came at no cost to me. Nonetheless, now that I have the upper and lower to build Frankenstein’s monster, let’s establish what other parts I want:

Barrel: I don’t care too much, as long as it’s cheap. My buddy’s dad had an old Olympic Arms post ban AR in 7.62×39 he wanted to get rid of, so I offered to do a barrel swap with him instead. I traded him my 16” 5.56 NATO barrel for his Olympic Arms 7.62×39 barrel, and boy is the barrel hideous. Heavy profile, no muzzle threads, and a pinned A2 gas block. I’m not really complaining too much, however, because I’m not spending a dime on it. Important information about the barrel can be found in the Testing and Tuning section of this article.

Cost: $0 (but minus one 5.56 NATO barrel – a Value Line barrel from KAK will normally put you back $110)

20170205_180540

Gas piston system: The piston of choice will be my favorite, tried and true, Osprey Defense OPS-416 piston system.

I chose it for two very important reasons:

  1. It is simple and lightweight. There are no return springs, special gas blocks, or regulating knobs. You plug it in and it works. That’s it.
  2. It was the piston system that resided on the 5.56 NATO barrel I traded to get the 7.62×39 barrel, so it was available (again).

Cost: $0 (normally about $300)

I’ve used this exact same piston system has already seen considerable abuse in two other builds. It’s cherry was broken on a 5.45x39mm upper I used to have, eating about 2000 rounds or so of dirty, corrosive, and cat-piss smelling soviet-era surplus ammunition. The piston system held up perfectly, gathering not one smidgen of corrosion in any of its anatomy. I then sold the 5.45×39 barrel and built a 5.56 NATO upper with the piston. As part of a 5.56 NATO platform, the piston saw roughly 2500 rounds and was not once cleaned. As you can see, it’s fairly caked, but still in fantastic condition:

A note about this piston system. The OPS-416 is definitely not the best choice for this platform. While the majority of AR-15 gas block support up to 0.125″ gas ports, the bore diameter of the piston stem (the tube feeding from the gas block into the piston chamber) is considerably smaller than 0.125″. So, if you need to open the gas port a lot on the barrel, you won’t get the full effect with this piston system (and trust me, you need all of the gas you can get with 7.62x39mm). I would highly recommend a different piston system for this platform. If you do decide to use the OPS-416, you will fight with a considerable lack of gas.

Also, make sure your gas block is PINNED when using this piston system. I may or may not have walked a set-screw affixed gas block out of alignment during a bachelor party with this piston system once… it happened mid bump-fire and was a total buzz kill. So far, it is the one and only stoppage I’ve ever had with it. The fix is simple, however: Buy a VLTOR low profile set screw gas block, a BRDE pinning jig (make sure you get the one for the diameter of your barrel), a taper pin reamer, and a gas block taper pin. Using a carbide bit, it can be done with a hand drill and takes, at maximum, 20 minutes to do from start to finish.

Bolt Carrier Cut: If you know anything about AR piston systems, you’d know that every goddamn manufacturer has their own opinion on where the op-rod strikeface should be on the carrier. Thus, I can’t just pick up a KS47 bolt carrier and slap on a piston conversion key. I searched long and wide for a solution, but came up with nothing…

ENTER DEUS EX MACHINA. I spent many of my lunches over four months trying to find a solution to my bolt carrier problem. Digging through a six-year-old thread, I happened to stumble across the mention of a company: Paladin Machine Services of Kershaw, SC. They have no website, but I found a sketchy looking e-mail address for the company on Teppo Jetsu’s website (I know, right?). With my fingers crossed, I sent an e-mail hoping for contact. Lo and behold, contact was made! They offered to cut my OPS-416 bolt carrier to work with AK-47 magazines at an exceptionally affordable price. One flat rate box and $50 later, my bolt carrier is in the mail to Paladin Machine to get cut. In case you want to keep your money local and not inundate Paladin with your requests, go to your nearest machine shop and give them this information:

ks47-bolt-carrier-cut-diagram

Yes, I hand drew that diagram and added the text afterward. Sorry for the smudges, but I’m about as coordinated with ink as a drunk trying to walk on a tight rope. Albeit, the drawing is an amalgamation of all information I could find on the proper cuts needed to use AK-47 magazines. I’m not a machinist or an engineer, so if you see something weird or not descriptive enough, please feel free to comment.

A week later, my bolt carrier returns, and the result is pretty good!

The bolt carrier works perfectly with steel AK magazines, but there wasn’t quite enough clearance for Magpul magazines. A little bit of Sharpie and some manual cycling of the action with a magazine inserted identified the problem. In the images above, you might notice a bit of shoddy tool marks that extend from the front of the bolt carrier to about 2/3 of the way down. That’s my super-duper redneck Dremel-gunsmithing skills at work. There were little ledges on the inside of both cut channels that were catching on the Magpul magazines, so I ground them down a bit to add clearance.

The purple on the second photo was left over Sharpie ink used to find out where the magazines were contacting the bolt carrier. It’s a pretty useful technique. You should try it sometime.

Once you are done modifying the bolt carrier, be sure to blue or Cerakote the exposed steel to give it some protection from the elements. I highly recommend a Cerakote Micro Slick coating because it goes on very thin and will just about seal the steel permanently. It’s good stuff.

Additional Bolt Carrier Cost: +$50. If you decide to go through Paladin Machine Services, know that if you have a nitrided bolt carrier, you will have to pay for the tooling also (because the one job will either wear out or break the tool). I never got a quote, so I have no idea how much that will set you back.

Hand guard: Apex Machining Gator Grips – The same free float tube I had on my 5.56 NATO upper. It works with the OPS-416, and that’s what matters to me. It’s all aluminum, so I usually wrap it in black self-adhering foam wrap to help keep my hands from getting cooked when shooting in high volume. Since the free float hand guard is supposed to go over the gas block, I cut off the sling swivel and sight post of gas block to make clearance. It’s crude, but it works. Be sure to blue or paint over the exposed steel after cutting so it has some protection from the elements. Also, be sure not to cut into the gas tube portion… I probably should have said that first. Finally, do NOT install the hand guard until after you’ve checked for proper piston function. Should you need to open your gas port, it would suck to have to take the hand guard apart AGAIN.

Cost: I already had them – $0 (normally $198)

Everything else: Whatever random parts I have lying around.
– Lower Parts kit: (normally $50)
– Buffer tube: (normally about $20)
– Buffer and buffer spring: (normally about $20 combined)
– Castle nut and end plate: (normally about $8 combined)
– Basic stock: (normally about $30)
– Sights: (normally about $60 combined)
– Firing Pin: OH, I almost forgot. Get a 7.62x39mm enhanced firing pin. It’s not a gimmick, they actually work. Upon firing, they protrude from the bolt face farther out than a standard firing pin, which will help IMMENSELY with steel cased and surplus ammunition. I have yet to have a single failure to fire with mine. (usually about $11)

Grand Total for me: $50. I was pretty dead set on not spending a dime on this platform to start with, but the bolt carrier ruined it.

Grand Total for an Equivalent Build: $1206.98

Quite honestly, it will cost you less if you just buy the gun pre-built, but it will even out in the end when you put a piston system and a free float tube on it.

Assembly

Funny enough, assembling the gun was the easy part. The lower is no different than assembling any standard AR-15 lower, other than you won’t need the magazine catch or the bolt hold open components, and the upper is just like a standard AR upper. The barrel went on, piston went in, and all parts fit perfectly. It started to look like a gun… well, an ugly gun.

The great thing about this platform, so far, is that there is really nothing special about the firearm assembly. It goes together just like any other AR.

You might have noticed the Devil Dog Concepts Hard Charger in the photos. They are not designed to fit on billet sized upper receivers, so it did not initially fit on my gun. I had to modify the Hard Charger to get it to fit and function. It works satisfactorily… that’s all I can say about it.

ks47build

Testing and Tuning

At this point, all that is left is to test function. If you have snap caps or dummy rounds, cycle them through the gun several times. You might find problems before you ever even make it to the range. Personally, this is where I confronted several frustrating obstacles.

The Bolt

You really don’t need to read this section unless you have issues with your bolt. I’m working with a super-duper old Olympic Arms bolt (that came with the barrel trade), and the fitment of the extractor is questionable at best. Honestly, if this was the quality of work they produced, it’s a real mystery how Olympic Arms continued doing business for as long as they did… My buddy told me that his dad had never even shot the gun that this barrel and bolt came from, and considering that the gas port was super clean and the bore was shiny and new, I can conclude that he spoke the truth. However, all of the finish on the chamfered portion of the bolt was completely gone. It looks like the original extractor design was not optimal, so instead of redesigning the bolt and extractor, Olympic tried to increase the slope of the chamfer, removing all of the finish in the process. Garbage.

08-bolt-extractor-polish

By the design not being ‘optimal‘, I mean that the extractor was leaning so far into the bolt face that the cartridges were hitting the flat edge on top of the extractor upon feeding, thus stopping the feed cycle and preventing the bolt from going into battery. I’ve looked at other 7.62x39mm AR bolts and their extractors sit nowhere near as far into the bolt as the one on this Olympic bolt does. I temporarily solved this by adding a little bit of tape on the ‘palm’ of the extractor, forcing it out a little bit. It still gets a 100% purchase on the case rim, so it doesn’t impede it’s ability to extract. So far, it has improved functionality marginally.

Moral of the story: Buy a quality bolt. PSA bolts are pretty good, so you won’t have much issue with theirs. If you buy elsewhere, make sure you do your research and buy the best you can afford. Liberty Gun Works and MGI make fantastic bolts; look into those. The MGI is actually made for ARs that use AK magazines… Just sayin…  Windham Weaponry also sells a pretty high quality bolt for about $55… I might just go with that.

SPEAKING OF BOLTS, if you plan to use Magpul magazines (and why wouldn’t you?), the bolt will murder the magazine’s feed lips. This is not a problem unique to piston driven guns, but all KS47 owners will encounter this. With AKs, you usually just apply vodka directly to the problem area and the issue fixes itself. I personally chose a less effective route and modified the bolt to remedy this problem.

05-bolt-lug-grind-1
Locking lugs ground down
06-bolt-lug-grind-2
No, that’s not rust, just blanket reflection
07-bolt-lug-grind-3
A little different view
08-bolt-scenic-view
Different lighting

As you can see, it’s a slight angle ground upon two of the bolt locking lugs. The below graphic will clarify which lugs I speak of: ar-bolt-face

Grind down the red area of the two locking lugs, and you should run clear of the magpul feed lips.

Those of you out there who have experience shooting ARs chambered in 7.62x39mm know that bolts are a weak point in the system. Due to its geometry, 7.62x39mm has a considerable amount of bolt thrust that just plain abuses the little locking lugs on AR bolts. It’s not uncommon to get cracked and broken bolt lugs, and grinding down two bolt lugs probably doesn’t help. I’m assuming that my bolt, made with obsolete Olympic Arms technology, will probably break at some point, so I’m already planning for contingency (like, buying a new bolt very soon). Modern manufactured 7.62x39mm bolts probably fare a great deal better.

Magazine Ingestion

Another frustrating universal quirk with the KS47 is that there isn’t anything in the design to prevents over insertion of magazines into the gun. That means if you put upward pressure on the magazine (like, say, you unwittingly use the magazine as a rest while you shoot from a table), the magazine will push up into the bolt carrier and prevent the gun from cycling. Fortunately for you, there is about 3/16″ of gap underneath the bolt carrier on both sides of the gun to allow for modification. You can see how I modified my upper to address this issue in my blog post PSA KS47 Magazine Fix.

cap-screw-ks47

Now that the gun feeds cartridges manually without choking to death, you can take it to the range… which is where the next hurdle arose.

Gas Ports and Your Piston

If you are using a piston system like I am, BE AWARE OF YOUR GAS PORT SIZE. 5.56 NATO has a SAAMI max pressure of about 55,000 PSI, while the 7.62×39 has a SAAMI max pressure of  of about 45,000 PSI. Since we are dealing with as much as 10,000 PSI less pressure than the 5.56 NATO, piston users will need to get a little cheeki-breeki with their port size. The factory port size on my 7.62×39 barrel was WAAAAAAY too small. The gun was incredibly undergassed upon the first test:

Sorry for the poor quality video, but 1000 frames per second is slow enough to see the parts moving. As you can (kind of) see, the bolt carrier only traveled about 1″ before returning back to battery. At the time of the test, the gas port size on the barrel was only 0.072″. For comparison, AK-47s have gas port sizes of 0.125″ up to 0.153″. Unfortunately, most AR gas blocks have port diameters no greater than 0.125″, so opening up the gas port past that diameter is completely pointless. I worked my way up, drill bit by drill bit, opening up the gas port and testing, but I had to go all the way up to about 0.125″ (1/8″ drill bit) for the gun to get a full stroke using lower weight buffer spring. It is key to note that every piston system will be different. I suggest you start by trying the stock port size (if larger than 0.072″) and increasing in drill bit size one by one until perfect cycling occurs. Don’t just jump up in size, unless you know what you are doing. This is why you should always test fire your gun before putting on your hand guards.

At this point, the gun feels like it only just barely works. Here is some unedited footage from the drilling and testing procedure:

Ammunition in the recorded test was Sellier & Bellot 124gr FMJ, but the results were the same with Tula 122gr FMJ and Tula 154gr SP. Since it wasn’t getting a full stroke, the lip of the bolt carrier (not the bolt) was catching the rim of the cartridge and locking up the gun. Also, the gun had very little recoil… like less than my 5.56 NATO did. Without a muzzle brake, the gun should have been kicking a lot harder, but it wasn’t. It was actually pretty peculiar. If the bolt carrier did go far enough back to feed the next round, it never fully reached the back of the buffer tube. While this technically mitigates recoil, it doesn’t give the action enough poop to properly cycle.

Anyway, to get the gun to cycle with the OPS-416, you will probably need to utilize a lighter weight buffer spring with a standard weight buffer, and a wide open gas port. In my ideal world, I want the gun overgassed with a heavy buffer spring. That way I can have the gun function with even the lowest pressure ammunition, but always slam the next round into battery with authority. Until I decide to install a different piston system, I will be stuck with my low weight buffer spring and undergassed gun.

Nonetheless, once you get proper cycling, the gun should shoot something like this:


All that is left to do is put the hand guards on and call it a day.

Let’s do a quick recap:

If you want an AR that accepts AK-47 magazines, the current market is in your favor. If you want to build one, Palmetto State Armory has you covered with just about everything you will need. Is their design perfect? No. But with PSA selling whole built guns priced on sale in the $700 price range, just about half of what everyone else is asking for theirs, it’s worth a little elbow grease. Perfection will require the modification of the upper to prevent the magazine from nosing up and binding up the gun, and modification to the bolt to allow clearance for polymer magazines.

Most importantly, if you want to build one using a piston system, the KS47 is probably your only option. To do so, you will only need to overcome the following hurdles:

  1. You will need to get your piston bolt carrier cut for AK-47 magazine clearance.
  2. Since 7.62x39mm is a lower pressure cartridge, you will have to open your gas port up for proper function with a piston designed for 5.56 NATO. Make sure you select a piston system that can be overgassed. Adams Arms should work (with a lighter piston return spring), and maybe a Superlative Arms.

I wanted a gun that shot 7.62x39mm ammunition, used AK-47 magazines, driven by a piston, but retained the ergonomics and modularity of an AR… and by George, I got one. It took a little bit of persistence, but it was by no means unfeasible. The gun has a lot of quirks, some good, some bad, but overall the platform speaks to me. What does it say? Well, a gentleman doesn’t share the specifics of his romances.

That’s it for now. Once I get the bolt situation figured out, I’ll run the gun hard and see what kind of impression it leaves on me. Expect a follow-up on this firearm in the future, because I’m in no way done with it yet.

Full review: PSA KS47

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11 thoughts on “Piston Driven KS47 – Chapter 2: The Build

  1. I have a set of recievers from PSA for the KS47. They sent me an RMA for an issue I’m having with it. The carrier seems to ride too high in the upper. My hammer puts a ton of pressure on the carrier. Did you have this issue with yours? I’ve tried 3 different barrels on the upper and I’ve tried 3 different bolt faces and 3 different carriers in each set up. The locking lugs are also having issues in each set up I’ve tried. Do you have any thoughts?

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    1. I haven’t had any issues like you’ve described. My hammer does ride on my bolt carrier pretty hard, but I rounded the top of my hammer to reduce friction. It isn’t too much of an issue now.

      As for the locking lug issues, what kind of issue are you having? Are they not locking? I know that there are two types of 7.62×39 locking lug setups, one is deeper than the other, but I’m not too familiar with it. If you can provide a little more information, maybe we can figure out what’s going on.

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      1. I think the main issue is with my upper and lowere receivers. The take down pins are incredibly tight. I have to squeeze the rear of the upper and lower together I order to get the pin out. I can’t push the front pin out until the units are pivoted apart. It just seems out of speck. I had a previous issue with a barrel from AR-15 performance. It had a misaligned piñon the barrel extension. And the chamber was wrong. The brass came out stretched way over standard 7.62×39 once fired brass and the shoulder was distorted after firing. Disappointing since they are supposedly a great company. The guy wouldn’t send me a replacement barrel and told me that some business and some customers just don’t get along.

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      2. Head spacing is correct. The alignment pin in the barrel extension was off so the locking lugs could not unlock.

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      3. Huh… That is some awful luck. Normally those barrel extensions are pre-drilled perfect using a CNC machine, so a misaligned hole in the barrel extension for the pin is pretty bad programming. I wonder how many of those bad extensions have made it into the marketplace…

        I’m assuming that the feed ramps are slightly crooked then as well. I’d still contact the better business bureau. That’s a pretty magical manufacturing error. You are due at least a little bit of money out of this one. Good luck to you, sir.

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  2. ive been building my rifle again. got a great barrel from Faxon. my bolt needed to be polished and it works more smoothly now. i have done a series of test fires and need to drill out the gas port as you found yours under gassed. i am also using a mid length gas system. how is your rifle running? have you done any further modifications?

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    1. My gun is now running pretty well. Other than drilling out the gas port (which you might find difficult if your Faxon barrel is nitrided), not too much more was done (barrel wise). You might need a slightly lighter buffer spring, but only slightly lighter. I have a carbine length gas system, so I’ve got the advantage of increased dwell time. Be aware of your gas block’s maximum port size as well. Don’t drill out the barrel port more than the gas block will allow. Most importantly, go shoot the crap out of it!

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      1. Gas port drilled out very easily. Made it fire better but I think I’ll be going up to 1/8″. I’ll try and find a lighter buffer spring as well.

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