Well coming off of the success of my 9mm AR15 build, and after realizing that my sks was no fun to shoot at 12" gongs at 565 yards (try even seeing the damn things with a naked eyeball) and running out of 5.56 ammo for my Bushmaster, I decided to build a 7.62x39 AR15 to help burn up this 4k+ round stash I have....
YHM 20" fluted, non threaded, chrome lined, rifle length gas system
YHM A2 Flat top stripped upper
YHM 12.5" customizable free float forend
Midwest Industries Low Profile Gas Block
Standard rifle length gas tube
NiB coated BC
Underground Tactical 7.62x39 fully assembled bolt
NiBx firing pin
Lower was an Aero Precision Stripped lower
Standard lower parts kit including the 8,000lb trigger pull (lol)
ASC 10 Round mag (bought from YHM)
6 position carbine stock.
During the first range outing it fired the Monarch and Tula ammo, and as expected, hated wolf (need to "enhance" my firing pin/bolt I know.) The Tula ejected but the BCG failed to strip the next round out of the mag. I then loaded only one round in the mag and it would fire and eject the round, but would not lock back the BCG. Monarch on the other hand cycled just barely, stripping the next round out, properly firing and ejecting and stripping the next, etc. However when I only loaded one round of Monarch, it would lock the BCG back but only so much that the bolt catch would catch the BC not the bolt face itself. With more than one round loaded, it was cycling JUUUUUST enough to run monarch reliably.
I tried lubing the hell out of the BCG to no avail. Swapped buffer springs with my 9mm AR carbine buffer spring (about 15% weaker) to no avail. (9mm incidentally operated fine with the heavier buffer spring.)
I gave up trying to get Tula to operate it and fired a few groups with the 123gn Monarch FMJ. Much to my surprise a 0.4" bore center to bore center group at 50 yards!! (The barrell is a shooter!!)
I have since inspected the gas system:
No burrs in the gas port, no obstruction in the gas block or tube, gas key on the BC seems a little loose around the end of the gas tube, but Im unsure how to measure 0.005" needed. I separeted the gaps on the three gas rings all by 120*, have since lubed the rings, moved the bolt into firing position in the BCG and attempted to blow into the gas key. Very difficult, but some air does get by. I MAY have origionally misaligned the gas block on the gas port, but not by much. Before the gas block was about 0.020" separated from the barrel's shoulder, now she is 0.030" and dead on the money lengthwise from the shoulder. I had also placed the gas block top dead center refrencing the fluting assuming the fluting done by YHM would be perfecty inline with the gas port. Upon closer inspection it isn't and was off perhaps 0.025-0.030". I repositioned the gas port so it should be on the money, but.....
The actual gas port on the barrell itself is huge - likely the same size as the port in the gas block. The smallest point in the system appears to be the hole in the gas tube by a few thousandths of an inch.
If this all doesnt work, Im unsure what to do next....perhaps NEMO's 300 black out buffer spring that's weaker than the stock buffer spring? Lightweight buffer?
ps - I should also state that I ditched the 6 position carbine stock for a full length A2 stock and associated buffer that upon disassembly has 4 weights in it. Im assuming I need a lightweight buffer next?