I tried my  7.62 x39 ar15 pistol build today. RRA upper (originally from a 5.56 pistol from them) with a new 10.5 inch barrel. gas port is carbine position. new low profile YHM gas block and tube, of which the installation looks good. lower is also RRA also from my original RRA 5.56 pistol. carbine buffer and spring i assume.. i use this lower on my 7.5 in 7.62 x39 upper and 7 inch 5.56  as well with no issues.

using both Wolf and Red Army Standard ammo, had all FTE's.  round is left about half way in the slot. feeding via manual bolt manipulation is fine. had a spare buffer tube spring , so we cut some coils off the spring to see what would happen. rounds would then weakly extract but it  would then  not load another round. (big suprise, i guess)

i am new at this but it sounds like it is under gassed. don't know what size the gas port is on the barrel. my next step i think is to take it to a Smith and check gas port size and then open it up as required. i am not sure what size the port should be for this round and barrel length. any help with this would be appreciated.

YHM gas block, did you leave a hand guard thickness gap before the barrel shoulder or did you push your block all way to the shoulder?  Some model YHM are designed to be set to the shoulder, and this is the first thing to check.  I assume it didn't lock back after the last shot from the mag, if you don't know you should fire one round with a mag in the well to see, and repeat number of times?  Do check your BCG key for leaks, easy to do if you have an air compressor.  Lastly if your BCG's extractor to see if it has an o-ring or not.

Oh and it is highly advised against cutting coils off the buffer spring.

make sure the gas block is on correctly and the tube isnt pinched.

sounds like its either loose or not installed correctly

thanks for the replies.  gas block was installed pushed against the shoulder it is YHM model 9383 .brand new gas tube and block. i have put on a couple before and and this went on no problem. i tried another bolt from another 7.62 x39 which runs fine in my carbine but no difference.  i will remove the tube from the block and check it before  any enlarging of the gas port.

just pulled the gas block and tube and rechecked it. it is fine. blew air through it no problem. no sign of restrictions.  position on the barrel looks perfect. don't have much in the way of tools here, but with what i have, i determined the gas port is slightly larger than .063 in.  .080 inches it is definitely NOT. it is  much closer to .063, which i believe is colt spec for a 16 inch barrel (5.56).  from my reading, i came across  threads by  guys with a 7.62 x39 10.5 inch barrel. they opened up the  ports from .094 up to .116 i believe, with success.  

i am asking you much more experienced builders how you might proceed in this situation. i don't have the tools for this job and i am very new at building AR's, so i may have to use a gunsmith. opinions, please.

My 10.5" 5.56 barrel has a .093" gas port hole.

Sounds like your rifle is under gassed.

Go to Grainger and get some drill bits that step up from your current size and start enlarging the gas port One step at a time until it functions well.

i may have to do that. the gas port specs are different on the 7.62 vs 5.56, though.  fortunately, the maker of the barrel is not too far from me and they are being great in that they are going take a look at the rig and figure out the issue. good guys at Tactical Ambush

thanks for the input

Does the bolt lock back after firing just one round in the mag ? If it does I'd try a #60 o-ring around the extractor spring, cheap and easy test.

I use the o-ring on my 5.56 and 7.62 pistols and it works great.

nope, the bolt does not lock back on a single round

swa, were in FL are you located?

You need to check the Key on your bolt carrier group for leaks, before drilling the barrel.

Have you looked into upgrading the extractor spring with something like the BCM kit? I am having issues extracting w/ my 7.5" 5.56 upper and that was a suggested fix. I ordered a 3 pack for $12 and will be testing it out.

thanks for the responses. i have tried two different bcg's, one from a 7.62 x39 carbine which is nickle boron coated and a liberty gun works bolt, and another bolt and carrier (7.62 x39)from RedX arms. no change. i am going to try a third Bcg that i have from a RedX arms 7.5 inch 7.62 pistol that runs like a clock and see what happens.  btw, i am in the boca area.

i am not going to drill the barrel. Tactical Ambush in Orlando makes the barrel. they are going to help me get it running right

I did not notice any comments about lube.  I'm sure it is well lubed, but if not...

SWA, you have a message in your IM box.

definitely well lubed. i tried a 3rd bolt carrier and bolt today. this bolt i removed the o ring from the extractor, no change. i then took the carbine buffer apart and removed one weight at a time until i was down to no weights at all in the buffer. still no change.
it's got to be under gassed and the port needs to be opened.  we will see

thanks for the inputs, y'all

Well a good news update on this build. I took the barrel back to Tactical Ambush, the maker, and Carlos and his team spent a lot of time working with this to check for solutions. not surprisingly, gas port size was the issue. it seems Tactical Ambush ports the barrels based on the use of good American ammo, not the low grade Russian Wolf or Red Army Standard stuff I shoot. it turns out .090 port size does the trick in my installation. I took it the range today and it ran like a top with good 1500 position ejections. Big kudos to Carlos and his team at Tactical Ambush for taking the time out of their busy day to get things right for me. BTW, this was at no charge to me. While i was there I took the opportunity to pick up an awesome chrome nickel bolt carrier. this thing is slicker than owl shit and i used in my &.62 x39 carbine today. very nice

Better too small than too big,then it would be like I cut it twice and it's still to short.

Kudos to them for fixing their mistake and kudos to you for using clear language in a troubleshooting thread.