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?

Any suggestions??

Which mags are you using?

 

:)

Incidentally, I went to an indoor range to play with it further yesterday.  HOLY ****!!  This thing makes FIRE BALLS and Im running a 20" barrel (not threaded for any type of muzzle device either.  (I need to find a 10" barrel in this caliber and make a pistol quick!!  Maybe even put a compensator on it and turn it into a real range pleaser!!!  LOL!)

But before even starting, I went bananas coating everything with rem oil...much better functioning, but still short stroking using the rifle buffer that has the 4 internal weights.

My next idea was to try replacing the 4 internal weights with the exact size dowel rod cut to match their length so as to keep the rubber bushings btwn the pieces of dowel and go from there.

Thoughts?



Thanks!  I do have access to some brass cased winchester 7.62x39, but it is going to take some work to find them  LoL!  I dont know why I didnt try my other BCG out of my 5.56 bushmaster...*DOH!*   Before I go out and purchase a wekaer spring and buffer, is my idea on lightening this current buffer to test out an ok idea?

What weight buffer are you using? I just finished my 7.62x39 build. It was doing what you describe yours is doing. I had a very heavy spring and an h2 buffer. It would short stroke. I pulled the heavy spring and replaced with a light spring kept the h2 buffer. It resolved the short stroke issue. Good luck.

Honestly I have no earthly idea - it's just the buffer that showed up with my A2 stock from brownells mid-ar15 scare...lol  And I was incorrect, it has 5 weights in it, not 4.  Ill put the 5 weights back in in place of the wooden dowels, take it to work to get an accurate weight on the buffer.  

With your build, what length barrel did you go with and how is the muzzle flash on it?

Ammo and/or possible gas block alignment,Mine did the same thing and I removed and reset the gas block and all is good.

Does anybody have experience with Model 1 sales Ar's in 7-62x39?

I don't know your build skills, but have to ask.  Are you sure the gas port is aligned perfect for the gas block?  

Was the barrel dimpled for the set screws or did you dimple the barrel yourself?  If was dimpled then it might be dimpled for a YHM GB (but all holes for the  front screw should be in the same place) as a MI GB it a little different.  If dimpled it yourself and used a fixture than it's probably correct.

Other than that I would look at swapping the carrier (gas key might be the problem) or the gas tube.

And last, but not least I'd look at the gas port size and google  what size other had to use to achieve reliability.  

If you pull your gas block after firing you should be able to see a circle on top of the bbl where the gas block is sitting. A small amount of carbon will build up. This should show you how far off you are. I had this problem in a 5.56 10.5" build recently. Best I can figure, the shoulder was turned with a free float hand guard and when I added an end cap it pushed the gas block out just enough to restrict gas flow. When I pulled it apart I could see the circle partially overlapped with the gas port. Once I readjusted it all was fine.

I just finished 3 of these builds, (mix & match parts) here are the things I did to make them all work

wolf hammer spring
honed down back of the bolt 0.01 shorter, standard firing pin extends  further
Had an issue with one build short stroking after 100 rounds, Bolt Carrier Key was loose, short stroking = gas leak or lack of gas  
honed down the feed ramps, purple dremel stone works great

the #1 thing was to cut off the legs of mag followers
on the 30 round mags I switched out the spring from a AK mag, or pick up wolf mag spring.

All of them work like a charm

Maybe an adjustable gas block?  Help tweak things a bit further.