I have built my own gas-inpingment M4 clone (16" barrel including compensator, and wrong gas system, so not really an M4) and an M16A3 clone (no select fire). I haven't had any problems, but my shooting conditions are certainly not close to combat.
I reload my own M193 and M885 ammo in addition to some really low grain, fast sh!t for personnel armor penetration (those pesky varmints).
So I am curious, regardless if a weapon is an AK or an M10, is stopping power that different? Is it worthwhile to hump heavier ammo?
One of our guys shot a dude ten feet away, four times in the chest. The guy ran away. Not, limped, not crawled, staggered...RAN. Of course the guy was wearing body armor - if you count a tshirt as body armor. A lot of those guys were hyping up on meth, injecting adrenaline, or on other drugs. You needed the heavier bullet to damage larger areas, or more rapid blood loss.
Not to mention the yaw is more violent on a 7.62x39 bullet than a 55.6x54 - it opens more flesh. At the time we weren't issued AP rounds, but 'lucky' for us some of the dead haji had some. (for those reading that want to jump on me about what "haji" means or political correctness - yes, I speak the language, and no I don't care)
Now, that I just said the heavier round is worth the weight, that doesn't mean all the latest whizzbang gadgetry is. The saying goes ounces equal pounds and pounds equal pain. The more you have to carry the harder it is, the slower you go, and the more fatigued you get. But, I would forgo half the other tacticool stuff, and even more of the other stuff in the pack in favor of 7.62x39 or 7.62x51 (.308).
One of our guys got shot in the back of the leg and the hole in the front was nearly large enough to stick your fist in. That cause a large amount of blood loss in a short period of time. I'd say pretty effective. For those wondering, he lived. Well he died, had his Last Rites read to him, and then he came back to life and lived.
Disclosure - I nearly had repeated jams of an M16 cost me my life. I am highly biased in favor of the AK. Yes, I will admit the M16 has tighter tolerances and groups and is more accurate at longer distances. You can hit your target pretty effective at 500 yrds with an M16 and you want 300 or less with an AK. That said, I have never been in a firefight beyond 200/250 yds, so I personally prefer a weapon with looser tolerances that can better withstand heavy sustained firing, mud, water, and sand and has a heavier round.
From a personal stand point (since I am not equipping an army or a Marine Corps), 7.62x39 is also cheaper than 5.56/.223 and the weapon systems are generally cheaper too - I was able to buy x2 AKs for what my buddy paid for x1 M4. And mine are tricked out with collapsible M4 style stocks, rail systems etc. Dollar for dollar cost was 2:1.
As an added aside, I decided that rather than buy a variety of different calibers, I would stick to 7.62x39, 7.62x54, 9mm, 8mm, and 12 ga. This gave me cost effective ammo I could stock pile in bulk while having a variety of platforms to shoot it. Stopping power is pretty good across the board, although I sacrificed a bit with the 9mm, but I can use that in a CCW as well as a carbine....and quite a bit cheaper than .40 or .45. The 8mm is for the Mauser my grandfather picked up at a beach resort on Normandy.