I have a 12.5 carbine length BCM barrel that has had the gas port reduced to 0.70. Every round or every other round fails to eject and the rifle tries to chamber another round while the spent case is still in the receiver. Obviously this causes a total malfunction.

The weird thing is that it only happens when one specific bcg is in the upper. This specific bcg works flawlessly in my other rifles and the rifle works flawlessly if I put in any other bcg I own.

I’ve tried all the different kinds of lowers, buffers, ammo, and magazines and no matter what there is always a failure when the one bcg is in the upper.

It was an easy enough fix to just swap the bcg out but I’m still curious about what the issue might be here.

Attached File

Attached File

Gas key bolts are still torqued properly and in one piece. I’m not seeing any areas around the key that show bleed off.

Check ejector tension and for brass shards in channel causing binding.


Don't know about the working in other rifles though.

Chamber needs to be polished and install a BCM extractor spring.

Extraction and ejection with the bcg in other rifles is perfect. Extractor is good to go.

Had the same deal happen. Only thing I could figure was that mysterious "tolerance-stacking" deal. I left both BCGs in the "other" guns and called it good. The one which would malfunction in the one gun was one of those fancy, shiny nickel-boron deals. The "replacement" was a nothing black phosphate. They were both happy in their new homes.

That’s what I’m doing now. Not sure where in the bcg, upper, or barrel has tight or loose tolerances.

The upper is a 3S679 marked FN upper, BCM barrel, and the bcg is a Toolcraft 1B1B6 marked carrier with a FN bolt. I threw a phosphate Wilson Combat bcg in there and it runs well. The Toolcraft/FN bcg is now in an Aero upper and Bushmaster barrel and that works well. Go figure.

One of those deals where my OCD demands answers but fortunately I quickly don't give a crap as long as things function in some alternative, acceptable manner.

Your extractor is letting go in that upper.

I understand the BCG works in other uppers, but that particular upper obviously needs more extraction power to work, due to many other variables in the assembly.

You can try polishing the chamber to reduce the load on the extractor, but usually when the chamber is rough, the case gets left completely in the chamber, not pulled out into the receiver.

Tony

What are the "many other variables"?

I've had only one AR that had a rough chamber to the point of marring brass 360*, end to end. Never failed to extract and after a bit of smoothing the chamber, the brass was unmolested. I've seen this phenomenon in a few others without the chamber holding on to the brass.