I'm definitely not new to building AR's or AR pistols. This is the first time I've had this issue: I built an 11.0" AR pistol and took it to the range for the first time today. The weapon cycled just fine, however, it was not locking the bolt back after the last round. I used a 20rd curved ASC USGI mag, a 20rd straight ASC mag, and a 30rd D&H USGI mag. The ASC mags had (different) ASC followers, and the D&H mag I had upgraded to the Magpul follower. I was shooting Wolf Gold brass-cased 55gr ammo. I tried each of these mags several times, but none of them held the bolt back with this particular weapon. I was also shooting several other 16" ARs, and they each held the bolt back after the last round with the same mags.

The BCG in this pistol is an AIM Black Nitride unit. I didn't swap the BCGs (from the 16" rifles to the 10.5" pistol), but I guess in retrospect I should have given that a try.
The buffer is a regular carbine buffer. Spring is nothing atypical either.  
The barrel was one of those KAK 'government overrun' deals for $100 back in May 2015. Carbine length gas system.
The gas block is an Armalite A2 Front Sight Clamp On w/ set screws. I rally took my time to properly align it.

Here's the rifle, and target from 75 meters. It cycled just fine, but it just wouldn't lock the bolt back.



Any ideas on how I can track down the hold-open issue and fix it?

I only ask because you didn't explicitly state it, but it does lock back if you pull the charging handle back on an empty magazine, right?

Good question above.

The ammo should be good to go, and that many mags should as well. It's decently safe to say it's the gun, thanks to the info you provided.

First thing I'd look at is ejection pattern. That helps to see if it's under or over gassed. Just an easy thing to look at. Either could cause that I suppose, though usually being undergassed would be the one I'd assume first. If it's undergassed, well then you get to start problem solving for that.

Otherwise you're looking at a weak/incorrect spring for your bolt release, an out of spec lower or bolt release that's binding, etc.

All that would be my first guess.

Make sure the bolt release is operating fine. If you can manually lock back the bolt on an empty mag then you know it is gas related.

Make sure to use plenty of lube and then step down buffer weights or spring weight until it locks back. If you are using a standard buffer and spring then I would imagine your gas block/ gas port are off.

That's a good question. Yes, it does lock back on those empty mags when I charge the weapon by hand.

So I guess this is all leaning toward it being undergassed, huh? Which means I should check to ensure the gas block is properly centered over the barrel ejection port. If it is properly centered, then I need to check the gas port hole size in the barrel, and possibly enlarge it a smidge. Is that all correct?

Mines undergassed too and it looks like mines leaking at the block/tube joint pretty bad.
Thoughts on sealing it up?

It sounds to me like it's just a touch undergassed.  Now, as to why it's undergassed.....

If you haven't already done it, try different ammunition (like maybe Federal XM193) to see if the problem persists.  Yes, Wolf Gold should be OK, but maybe what you have is part of a wonky lot.

It's normal for gas to leak out between the block and tube.  It's most pronounced when the build is new.  After a few hundred rounds or so, the gap tends to get caked up with carbon, thus sealing it better and reducing leakage.
This, by itself, shouldn't cause short stroking, presuming that your block and tube are both within tolerances.

If everything else checks normal opening the barrel port  one drillsize size larger should take care of the poroblem.

Some of my AR's would not latch with my then brand new 30 rd Gen2 mag pul mags. I use 20's now as they work in everything and I can shoot prone easier without the large mag sticking down.

That's where I'm at. I checked everything - the residue showed that the gas block hole was centered over the barrel port, so I opened the barrel port size one bit up. I'll take it to the range on Sunday to see how that change worked.

How many rounds do you have through it?  

I hand cycled my two builds a bunch (>>100 cycles) with no lube to break in the wear surfaces.  Both were definitely tight at first.  Could be your rifle just needs more cycles.

Total, I now have about 100-120 rounds through it. It cycles fine when it's running. Just not locking back after the last round in each mag.


I took it to the range again today after removing the gas block, verifying it was centered over the barrel gas port hole. While I was at it, I enlarged the barrel gas port hole to .081", then reassembled everything. I also put new Magpul USGI followers in each of the magazines. One of the 20rd ASC mags did not have an anti-tilt follower, so I changed them all to the same follower.

I had more oddities during today's shooting. It was not picking up the first round from any of the magazines. This was both via hand charging the rifle, or by Locking the bolt back, inserting a mag, then pressing the bolt release. Eventually, I swapped BCG's with another AR I had with me at the range. This issue then went away.

I still had issues with the rifle locking back after the last round in the mag. A few times it did lock back, but most times it did not.

My next troubleshooting will be to do the following:
1. Remove the BAD lever to see if this changes anything.
2. Bring another one of my pistol lowers and shoot the same upper with that different lower.

I'm really at a loss here. I've built over 20 other ARs of various barrel lengths and I've never had issues with any of my other builds (including a 7.5" pistol which runs perfectly).

Anyone have other troubleshooting ideas?

There's a little tidbit you could have mentioned right off the bat. Have read any number of folks talking about trouble from it.

- OS

Yep, really helps having all the info up front.  I had one upper that didn't play well with a BL.

Here's something to check, when you manually pull the charging handle all the way back, where does the face of bolt stop in relation to the back edge of the ejection port?  It should stop about 1/8" to 1/4" in front of the back edge of the port.