Bolt not locking on last round

[ballsout]

I have browsed a couple write ups on this issue after scouring the net. Need help going over the best method or order of elimination for diagnosing the issue. As the title says my AR starting having issues not locking the bolt back after firing the last round. Probably in total 140 rounds or so with this setup. Didn't notice this issue until 50 or so rounds ago.

Set up
Upper: BCM BFH Middy ELW 14.5" w/ BCM upper receiver group
Lower: Noveske Gen2 lower with G&R lower parts kits, LMT buffer tube, H buffer, SSA trigger
Ammo: Federal XM193, All brass ejects at 3o'clockish

1) Is it the magazine? I tried 5 different pmags, different generations and colors. All same issue with bolt not locking back on last round.

If it is not the mag what should I look to next?

Bolt catch/spring, buffer, gas key? What should I try next to be efficient in fixing this?

[Joe R.]

First check is mags. You seem to have covered that. Next check is are you losing gas. Is the gas key staked? (Since it's a BCM gun I would think so) Are the gas rings good? If the gun was working before these are your normal failure points. Check the bolt catch to see if it might have broken...not normally an issue but certainly possible. Last is check the buffer spring. Is it worn (too short)? Should not be buffer unless you've changed it out before the issue started.

How many rounds through the gun?

Ammo could be a possibility, but again if it worked with this ammo in the past it is probably not the issue at this point unless you got a bad lot.

[MistWolf]

1) Lock an empty mag in the mag well. Rack the action completely to the rear. BCG should lock open. If not, it's usually the bolt catch. It could also be the mag. Try a couple of different mags

2) If the rifle passes the above test, load a magazine with a single round. Lock the mag in the mag well, load the rifle and fire it. The mag should lock back. If not it's usually due to short stroking. It could also be an out of spec bolt catch or magazine. Try a couple of different magazines a couple of different times

[ballsout]

Bolt catch operates normally with empty mag. I tried loading 1 round in each mag. Same result with bolt not locking back after fired round.

[ballsout]

1) Lock an empty mag in the mag well. Rack the action completely to the rear. BCG should lock open. If not, it's usually the bolt catch. It could also be the mag. Try a couple of different mags

2) If the rifle passes the above test, load a magazine with a single round. Lock the mag in the mag well, load the rifle and fire it. The mag should lock back. If not it's usually due to short stroking. It could also be an out of spec bolt catch or magazine. Try a couple of different magazines a couple of different times

Bolt catch operates normally(locks back) with empty mag. I tried loading 1 round in each mag. Same result with bolt not locking back after fired round.

[Clint]

The other static check is total travel.

Pull the charging handle all the way back and hold.
Verify the front edge of the bolt lines up with the rear edge of the ejection port.

Assuming that checks out,

Was this a factory assembled upper? What about the BCG?

[ballsout]

The other static check is total travel.

Pull the charging handle all the way back and hold.
Verify the front edge of the bolt lines up with the rear edge of the ejection port.

Assuming that checks out,

Was this a factory assembled upper? What about the BCG?

Yes a complete BCM upper receiver group assembled upper with BCG. The bolt does not appear to be completely flush with the ejection port but neither does my 6920 when i checked on there. Maybe a few millimeters from flush. After manipulating my bolt catch it seems "sticky". Surprised I didnt notice sooner. Could it be as simple as a sticky bolt catch?

[pstk5088]

I had a very similar problem, ended up being the bolt catch spring was weak. It would rarely lock back on an empty mag, but it would lock back periodically in the middle of a magazine. I was blaming it on magazines at first, then noticed the bolt catch seemed off. Replaced the spring, no issues since.

[sbrown3]

Try a standard carbine buffer and see if that does the trick. A buddy of mine just got the same upper as yours, but also a BCM lower, his came with a standard carbine buffer.

[JB2000]

I recently built a lower and the magazine catch was dragging in the slot in the lower. If not the spring that could be an issue too.