PDA

View Full Version : Last round bcg lock back fail


WhoDares
02-22-2011, 6:16 PM
I was at the range yesterday to zero my two AR uppers. One is a BCM 16" middy with a gov't profile barrel, the other is a no name(although possibly an FN upper and barrel) A2 20" flat-top rifle-length upper. It's the first chance I've had to fire the rifle upper, the fourth or fifth time for the BCM. I'm using the same lower (S&W M&P15) and the same bcg (BCM), just swapping everything when I switch uppers.
Ever since I first shot the BCM upper, the bolt fails to lock back after the last round every time I shoot it. Never any other issues, just that. With all kinds of brass cased ammo. Using C-Products 10rd mags with Magpul followers. I have a Magpul ACS with a DPMS(I think) buffer & spring.
Here's the thing; that 20" upper had no problems locking back the bcg after the last round...it worked splendidly(YAY!) Used all the same mags and as far as I could tell the same lot of PMC 55gr .223 fmj. So why does the rifle lock it back when the 16' middy carbine...a BCM, no less... doesn't, all things being equal?
I realize this thread is similar to: http://www.calguns.net/calgunforum/showthread.php?t=399423, (see I can use 'search';)), but my issue has a slight tweak.
Thanks in advance!

Kingpin.SC
02-22-2011, 6:27 PM
What buffer tube/buffer/spring are you using?

gemini1
02-22-2011, 6:59 PM
So did you have the same problem with the BCM even on the first time you've shot it? What type of buffer do you have? if its heavy, then try the lighter ones. I would do this first before checking the other parts. The rifle lenght works fine because it has more than enough pressure to push the BCG back against a carbine lenght spring/buffer tube.

WhoDares
02-22-2011, 7:25 PM
What type of buffer do you have? if its heavy, then try the lighter ones.

The buffer itself has no markings on it. Where do I look for the mark?

Also, the buffer tube/buffer/buffer spring may be DPMS...I just don't recall.

gemini1
02-22-2011, 7:54 PM
You probably have the regular carbine buffer if it has no markings.

http://www.biggerhammer.net/ar15/buffers/

Did you have this problem even the first time you shot the rifle? is your BCM carbine lenght? check your gas tube/block for any leaks, sounds like its not giving enough pressure to push the bcg.

WhoDares
02-22-2011, 8:16 PM
Did you have this problem even the first time you shot the rifle? is your BCM carbine lenght? check your gas tube/block for any leaks, sounds like its not giving enough pressure to push the bcg.

Yes, the problem has been there from the beginning. My BCM is a 16" Midlength gas system. How does one "check the gas tube/block for any leaks"?

gemini1
02-22-2011, 8:39 PM
Whoa, you have a middy and your having problems with a carbine buffer/tube/springs? Have you tried to cycle the rifle on an empty mag and does it lock back? Check if theres any excessive carbon build up at both ends of the gas tube. I also have a BCM middy but I never had any issues using carbine buffer. Did you get this upper brand new? you may want to email BCM about the issues and have them fix the problem.

WhoDares
02-22-2011, 8:45 PM
Have you tried to cycle the rifle on an empty mag and does it lock back?

Alas, it does lock back when I manually cycle the gun. Guess I need to call BCM. Drats

Hozr
02-22-2011, 8:59 PM
Check the bolt catch. Mine was a little too snug in the receiver and wasn't moving fast enough to catch the bolt. see if it moves freely with some oil.

WhoDares
02-22-2011, 9:16 PM
Check the bolt catch. Mine was a little too snug in the receiver and wasn't moving fast enough to catch the bolt. see if it moves freely with some oil.

I initially thought that was it, but everything functions properly when I shoot the 20" rifle upper. It (the bolt catch) seems to move fine with that upper. Could there be a reason the bolt catch moves too slow with the midlength upper and is fast enough with the rifle upper? And I have oiled it just to be sure.

SgtJT27
02-22-2011, 11:00 PM
From what everyhing you described, it sounds like the gasblock has leaks or blockage; enough not to push the BCG all the way back.

WhoDares
02-25-2011, 7:31 PM
Thanks to all for your thoughts on this matter.
I want to check the gas block and the gas tube for leaks and/ or blockages. Is there a way to do this without the use of special tools and/or compressed air?

killshot44
02-25-2011, 8:32 PM
Old trick.....
Block the chamber tightly. Light a cigarette/cigar and with a lungful of smoke, blow hard into the muzzle. Keep it pressured and look for smoke weeping out.

IIWM, I'd box it up and send it to BCM.....

dieselpower
02-25-2011, 9:26 PM
From what everything you described, it sounds like the gasblock has leaks or blockage; enough not to push the BCG all the way back.

The Gas Key is leaking
The gas block is leaking

Old trick.....
Block the chamber tightly. Light a cigarette/cigar and with a lungful of smoke, blow hard into the muzzle. Keep it pressured and look for smoke weeping out.

IIWM, I'd box it up and send it to BCM.....

Do this.

To all of you who said it didnt have enough power to cycle properly...please dont give advise unless you know what you are talking about. If a 20" upper cycled properly with the same buffer system...its not the buffer system.

1)gas tube
2)gas port
3)gas block
4)gas key
5)gas key bolts


And everyone please store this in the memory banks.. BCMs fail right from the factory too....at the same rate as any other manufacturer.

pacrimguru
02-26-2011, 12:47 AM
two suggestons:

1) check your gas rings to make sure they are aligned properly. (that is, gaps in gas rings not aligned.)
2) make sure your gas key is on tight and staked