Previously reliable AR suddenly not cycling.

[Glock9mm1990]

I agree with what others have suggested try a different BCG and see if that fixes the problem. If however that doesn’t fix it I would email BCM if I were you and send the upper back because there is no reason a BCM upper should stop working after only a few thousand rounds, I believe they do come with a lifetime warranty to the original owner. Let us know what the problem turns out to be and what the fix is.

[TomMcC]

I don’t remember the exact specs for the gas tube, but I think the mushroom for a new tube should measure approximately .180 and problems will start at around .170 or thereabouts. Visual inspection is going to be difficult. The tube can also erode at the gas port, which is not easy to diagnose without removing it.

Thanks for the info.

[Circle_10]

The culprit would seem to be the bolt carrier itself.
Took the gun back to the range after work today, racing against the clock since my local range doesn't allow shooting after sunset, which currently comes at about 4pm.
Just to double check, I first fired the gun with the same carrier. Identical results to yesterday. Weak ejection and total failure to feed rounds out of the magazine.
Took the bolt, firing pin, firing pin retaining pin and cam pin out and put them in a second BCM carrier. Gun functioned normally, locked open on the last round, healthy ejection. Fired about 35 rounds and had to pack up due to sunset.
So I must either have a gas key that's loose after all or that has some kind of obstruction in it that I haven't yet detected.

[Circle_10]

Using a Wheeler Fat Wrench on the gas key screws revealed that the front one was in fact loose. So I'm assuming this is the source of the short-stroking.
The rear screw is properly torqued to at least 50 inch pounds.
So is this a "send it back to BCM for repairs" thing or a fix it myself thing?

[AndyLate]

Contact BCM. Work with them to resolve the issue to your satisfaction. My 2 cents.

I am glad you isolated the problem!

Andy

[Circle_10]

Me too, the screw was not overly loose, but it was just enough I guess. Doesnt take much apparently. After ruling out the gas tube as the source of the problem at the range earlier I decided to really take a hard look at the gas key. The loose screw was torqued to less than 35 inch-pounds by time I noticed it. It would seem the staking was insufficient. I'll get in touch with BCM and go from there. If it turns out I have to fix it myself it's not a big deal. The carrier is actually one I bought in 2009, aside from a carbine class in 2013 the rifle it was in didn't see much use until this fall when I really started shooting it regularly.

[MQ105]

Test the BC after you tighten the bolt before you declare victory...

[Glock9mm1990]

Let us know how BCM responds.

[Circle_10]

Test the BC after you tighten the bolt before you declare victory...

I will definitely be testing it before trusting it in another gun. At least it's a fixable problem. For now, I've got the other carrier in my rifle.

[LMT/556]

Had the same issue with a C4 carrier, torqueing the key fasteners to spec and re-striking the stake corrected the issue and it's run great since even with sloppy loose rings. This BCG was sent back to mfg in the malfunctioning upper and returned loose.