Gentlemen...

My buddy picked up a ARx39 upper in the EE a while back because he liked the one I was running and wanted one....and no stones are being thrown at the seller here....this is not a misleading or false listing issue so don't read into this.


It is a YHM with a 20" diamond flute barrel with no flash hider, rifle length gas tube and the rest of the stuff as shipped from YHM.
The only difference that I can see from the Entry Level is that it has a different 4-railed gas block...it is not the two-piece nor the other set screw type.
It clamps on but has two set screws that tighten from the side and not the bottom thus clamping the block onto the barrel...not stamped YHM like the other two I own.
It is a really sweet looking setup but it has one little problem...it won't cycle.

It will fire a round and eject but it will not strip and load the next round.


Here is what I have tried to assist him....

Tried the upper on three different lowers.
- another carbine lower with a standard buffer/spring
- another carbine lower with a ST-2 buffer and standard spring
- A2 lower
- A2 lower with heavy buffer

Tried swapping out the BCG with my own
Tried swapping just the carrier using the YHM bolt group
Tried swapping in a FA carrier and his bolt group

Going heavier created stovepipes or a failure to eject after the round was fired so I looked to the gas system as the original weight/BCG/buffer (&spring) was not the issue.

I removed the block and tube to make sure it was not installed upside-down.
I ensured the tube was clear from port to end.
I made sure the tube port lined up with the vent hole.
We tried two different ammo manufacturer types of ammo with the same result.
We tried a 5-round and a 30-round mag...no joy.


So I think it is a gas issue but I am not sure how to go about fixing it.
He did not get it from YHM and although it was not stated, I am not sure the gas block/tube came from YHM nor not sure it matters.
I've never had an issue like this that wasn't fixed by correcting my stupid mistake so I am stumped on this one.

Thank you in advance.

You are undergassed. I agree with Direct Drive that the gas block is junk. Replace that with a quality block. I would put a standard carbine spring in and with standard buffer and drill out the gas port if still having issues after replacing the block. Its easy to do.

Are there 2 indentations on the case from were it makes contact with the barrel extension?...If so, that is most likely your problem...Sharp edges of barrel extension lug slots will need to be edge broken and smoothed over....I had this same issue with new 7.62x39 upper and this solved my feeding issues.