In a previous post, I alluded to the canonical article on AR 7.62×39. That is this article, an excellent resource by a guy who goes by Major Pandemic1. It identifies two issues with AR functioning in 7.62×39 rifles: cycling, which I will use to mean exclusively the movement of the action, and feeding, which I will use to mean the process by which ammunition is stripped from the top of the magazine and pushed forward into the chamber. These two issues are, at their base, related, and Major Pandemic hits upon the solutions pretty quickly, along with one which I believe to be superfluous. This is because he hit them in the wrong order.
It’s impossible to claim an AR-pattern rifle is unreliable without first making sure the gas system is functioning as designed. Thanks to parvusimperator’s recent book acquisitions on and general encyclopedic knowledge of the history of the scary black rifle, I have a few instances to cite. First on the list are the M16 and M16A1, where a change in powder from stick-type to ball-type yielded a change in pressure curve. The result: unreliable functioning2. Rounding out the list are your various carbines, from the CAR-15 in the 1960s and 1970s to the M4 carbine through to the super-short Mark 18/Close Quarters Battle Receiver, all of which fall prey to another issue: changing the barrel length without changing the gas system. When you shorten (or lengthen) a barrel, you decrease (or increase) the dwell time. That figure, the measure of how long a bullet stays in the barrel (and barrel pressures remain high), combines with the pressure curve to determine where the gas port should be placed, and how large the gas port ought to be.
Fortunately for 5.56 NATO shooters, the hard work has already been done, and barrel manufacturers have the sizes and locations pretty much figured out. Unfortunately for we 7.62 Russian Short shooters, the same body of work is not yet done, and shooting a different cartridge with a vastly different pressure profile is the very definition of messing around with the gas system. When doing your function tests, be prepared! If you run into cycling issues, try a lightweight buffer and/or a lightweight spring. (I believe the received wisdom is to try them in that order. Parvusimperator will correct me, if not.†) If that doesn’t suffice, you may have to increase the size of your gas port. With a drill, I mean, and a bit. Don’t increase the size too fast: my 7.62 AR functions perfectly with a factory gas tube size of about 1/12″. If you get to larger than 1/8″, you’ve almost certainly done something else wrong. A bluing pen (if you’re using a blued or nitrided barrel) should give you some finish around the gas port.
Next, and obviously, get dedicated magazines. The 7.62×39 cartridge is obscenely tapered, and only magazines specifically designed for it will push your cartridges up in front of the bolt carrier proper-like. This isn’t .300 Blackout; it’s not shaped anything like a 5.56 round, and any magazine not designed to feed 7.62×39, in a word, won’t. Midway’s AR-Stoner brand works well, and they won’t break the bank.
That brings us to feeding issues. By this point, you shouldn’t have any. If you clicked through to Major Pandemic’s article, you’ll see that he decided to dremel out the divider between the feed ramps. Granted, Bushmaster’s successful, functional 7.62×39 AR took the same tack, but my suspicion is that they made the same mistake other manufacturers, plus innumerable individual builders, made: an undergassed or overbuffered gun leaving magazines insufficient time to fully feed the next cartridge before the bolt carrier returns.
If and only if you simply can’t get your rifle to function correctly, you may consider a few courses of action before breaking out the dremel. Different magazines may help. It’s a bit of a shame that you may have to match a specific brand of magazine to your rifle, but if you were expecting rock-solid reliability with any equipment you care to find, you should have bought an AK. You might also try shooting it more. There’s a break-in period to any gun, and springs and lubrication may not take at first. If you do resort to dremeling and you have a barrel with a finish, don’t forget the wee touch-up pen to get your corrosion resistance back.
I think I’ll close with one final note on barrel selection. Faxon Firearms, who I’ve mentioned before, seem to make an excellent product with a gas port able to run just about any example of our favorite Russian intermediate cartridge without issue. I recommend their product. If you ignore that recommendation, I would at least suggest you look for a nitrided/Melonited barrel. Nothing else really makes sense for 7.62×39: presumably, you’re going to want to shoot cheap steel-cased, bimetal-jacket ammo, and a harder barrel helps to offset the additional wear you get from that choice. Combine that with the much lower velocities you have to work with compared to 5.56, and you may be surprised at the barrel life you end up with.
Then again, you may not. As you may have noticed, we don’t have sponsors around here, so I’m engaging in the most rampant of rampant speculation, not being able to afford 1) a second barrel to beat up and 2) the 20,000-40,000 rounds of ammunition it would likely take to really blow the first item out. As always, we’re curious about your experiences with 7.62×39 ARs, dear reader, so leave a comment if you have any.
1. I’m hardly one to talk about strange monikers, though.
2. I found some rather morbid documented accounts of US soldiers found dead next to malfunctioned, torn-down M16s, suggesting the men were killed while trying to fix their rifles.
† I would very strongly recommend messing with buffers, buffer weights, or even a reduced mass bolt carrier before touching the buffer spring. There are plenty of options out there to reduce the mass of the operating components if you want to go that route. Clipping spring coils is a good way to get plenty of malfunctions. -parvusimperator