Making the 7.62x39 Work Reliably on the AR15Running the 7.62x39 in the AR15 platform has gotten somewhat of a bad rap. Read through forums and you would think people are a little bi-polar with one post saying they could never get it to work and the next post noting they bought one off the shelf or built one and never had an issue. Kinda like people who love or hate the 300 Blackout.
Just as I was a fan of the previously cheap-to-shoot 5.45x39 round, I am a fan of the now cheap 7.62x39 round. Prices on that 5.45x39 round shot up a bit after they prevented cheap import surplus rounds, however the 7.62x39 round is still cheap and has been even through the great ammo crisis of the last ten years. I am not kidding… I could actually walk into Cabelas and buy whatever quality I wanted of $6 per 20 boxes of steel cased AK-47 rounds. I have a lot of time behind my AK-47 at this point thanks to 7.62x39 ammo availability. Currently, I am seeing that the venerable 7.62x39 round is still consistently 20%+ less expensive than .223 for plinking rounds. My thought was why not complete a build that could save enough money on the first 2000 rounds to pay for the entire 7.62x39 AR15 upper… you know, if I can get it working.
WHY A 7.62x39 AR15 UPPER?
Some people will ask why you would ever want a 7.62x39 AR15 upper. Beyond the cost savings facts, the 7.62x39 is actually a more versatile round in many respects than the 5.56 Nato round and has proven itself as a better deer and hog round by AK47 hunters. Heck, the 7.62x39 round is basically just an up-powered 30-30 round which is arguably the most successful deer hunting round in America. The 7.62x39 delivers more energy than the 5.56 Nato/.223 Remington or 300 Blackout out to the 200 yard mark which is where over 95% of training, plinking, defensive and hunting shooting is done. Many will say why not just use the 300 Blackout? Well from my experience the 300 Blackout round shares many of the same functional reliability problems as the 7.62x39 round running in an AR15 unless either caliber are well tuned. Yes... yes, I know your 300 BO rifles runs great, mine needed tuning to run both subsonic and high velocity rounds. Add in the 300 Blackout is still $1.00 a round compared to $0.40 per .223 round and $0.30 per 7.62x39 round. The 300 Blackout has its place, however a working AR15 7.62x39 upper can deliver more power for less money... you know, if I can get it working.MAJOR FUNCTIONING ISSUES OF THE 7.62x39 IN AN AR15
The most significant problem with the AR15 format running AK-47 ammo has been feeding and functioning. There have been a lot of theories on what exactly is needed to make the round run in the AR15, however usually it seems to come down to recommendations to start cutting on magazines and I don’t think magazines alone are the culprits. I wanted to see if I could figure it out for you my Pandemians and in the process have some fun with this awesome round.Every possible problem I could have experienced with this build, I did and nothing came easy. I placed an order with Delta Team Tactical for a $139 16” 7.62x39 Melonite Barrel with a Carbine length gas system and their complete $139 7.62x39 Phosphate Bolt Carrier Group. Although inexpensive and high quality products, the company could not have screwed up my order more if had been somehow intentional. A month later and I was still working through them sending me the wrong thing… again, but problems were eventually resolved. I plugged the barrel in a Anderson Manufacturing blemished $40 upper and then used a new Parallax Tactical M-Lock forend. The only real high dollar components on the upper were a Fortis gas block and a Lantac Dragon comp to take the bite out of the heavier recoiling round. A Lucid red dot topped the upper, however it would be months before I was able to really get to using it. For the majority of testing I used my WMD Beast lower.
GETTING THE 7.62x39 TO WORK IN AN AR15
Once assembled, I ran to the range excited to function test the build. Testing centered around the inexpensive ammo with a couple steel cased Wolf and Cabelas Herter's 7.62x39 rounds using Magpul Pmags and some C-product 6.8 SPC mags which some had noted would work just fine if used with just a few rounds... they didn't. The upper would not feed more than one round at a time and even then seemed to still jam the first round on the center of the M4 feed ramp and in many cases would not even fully extract. When the gun did cycle, the gun was also short stroking big time and was nowhere near approaching the point where it would lock back on the magazine. Problems? I had problems. Surprisingly the only problem I did not have was detonation. Many have noted that you need a special firing pin, however at least with this standard Mil-Spec lower trigger, there were no detonation issues. 
After the first time out, I realized the gun was way way way under gassed with a far too small barrel gas port hole, I also needed a dedicated Ar15 7.62x39 magazine for testing, and needed some work on the barrel's feed ramps.Many range trips later of test, fail, tinker, and repeat, I had polished the feed ramps to a mirror finish and drilled out the barrel gas port to a huge 1/8 (.125") size and suddenly the gun was locking back and at least trying valiantly to pick up the next round in the magazine. Jam city baby even with the C-Products dedicated 7.62x39 magazine feeding FMJ, but I really could not fault the magazine. There was more wrong here than magazine issues and what my aggressive feed ramp polish would resolve. That split feed ramp was an issue.
What I found was that most people had figured out the gas port size problem, however feeding was always an issue and I saw only one company address it completely and that was Bushmaster. Bushmaster offered a 7.62x39 rifle and from all accounts it fed and functioned like a dream. 
The main design difference in these rifles were oversized gas ports and they featured a single large feed ramp which in essence removed the split between the M4 feedramps to just form one giant ramp. This made sense because due to the tapered case of the AK47 round the round would angle toward the center of the feed ramp when the bolt attempted to pick up a round and guess what is in the middle, yep the split in the middle of the M4 feed ramps. So, I whipped out the Dremel and got to grinding away the split. Sure I took a couple deep breaths before I basically trashed a perfectly good barrel extension, however in my mind it had to be done. It worked and worked perfectly. In fact now I can fed this upper with pretty much any magazine, however the C-Products dedicated 7.62x39 magazines delivered the best high capacity performance. I would expect any unmodified Stoner and ACS clone 7.62x39 magazines to deliver the same performance.I have now put twenty full mags through this upper running on a variety of lower receivers including the WMD Guns Beast lower and it has performed outstanding. In fact I have had zero functional issues in those 600 rounds other than the very occasional bad round which is expected with inexpensive ammo.
WHY HAS SOMEONE NOT FIGURED THIS OUT BEFORE?
Well they have. Bushmaster did, however it was just not very popular and they discontinued it. From what I can tell the single large feed ramp idea died with the discontinued product line. The prevailing issues which would solve all the major issues are that the gas port sizes are almost always too small for the 7.62x39 round and that the split M4 feed ramps only cause jams. If barrel manufacturers delivered a single large feed ramp and 1/8" gas port hole in the barrel, almost all the issues would be resolved. Add in some dedicated 7.62x39 magazines and you are good to go. 
If you have feeding issues, Dremel out the middle of the M4 feed ramp and its likely any feeding issues you are having will disappear. If your bolt is not locking back on the last round, you probably need a larger gas port. In my case I needed to enlarge mine to 1/8", you might be able to get away with a smaller port depending on your build. This process is as simple as removing the gas block and then carefully drilling out the gas port hole to a larger size. I would start smaller and slowly increase the size.FINAL THOUGHTS
Apparently there are many people who have been able to just buy working 7.62x39 uppers slam in a dedicated 7.62x39 magazine and start rocking the round, however the vast majority of folks have issues of some sort. Hopefully my tips and modifications of a ground up build will allow you to build and enjoy a problem-free AR15 7.62x39 upper and save some cash in the process as you enjoy a cheaper to shoot AK47 round. Next up? How about a 7.62x39 AR15 pistol build.
SOURCES
Delta Team Tactical - http://www.deltateamtactical.com/
Lantac - http://www.lantac-usa.com/
Parallax Tactical - http://parallaxtactical.com/
WMD - https://www.wmdguns.com/