HELP HELP AR Trouble.

[Ogallala_1]

Okay my buddy has a bushmaster vamiter 24 inch stainless about 1k worth of rounds down the tube. It seems that the gas block has slid forward just a very little. So we loosened the set screws and tapped it back. You can fire about five rounds and the tube slides forward again. Normally I would say no harm no foul but, this gun wont cycle. We have cleaned oiled recleaned no oil checked for carbon changed mags changed bolt groups changed lowers and still this upper wont function. It appeares to get worse the warmer it gets, new or reloads same situation, we are stumped. It wont lock bolt back when mag empty and wont put new round into the chamber its like the bolt isnt coming all the way back when fired but we cant figure out why. Any help is greatly appreciated.

Thanks John

 

[KYS]

Re: HELP HELP AR Trouble.

Send to a gun smith or back to Bushmaster. Or you can buy a new gas block and have pins drilled in

 

[KUSA]

Re: HELP HELP AR Trouble.

You need to make sure the gas block is on correctly. If it is not then obviously it won't cycle. I wouldn't worry about the rest of the rifle until I had the gas block and tube installed properly.

 

[tt350z]

Re: HELP HELP AR Trouble.

Is the barrel dimpled for the set screws?

It's definitely short stroking.

 

[lwrkeysfisher]

Re: HELP HELP AR Trouble.

The rifle's gas port is approx. .089", movement of approx 100th of an inch is likely to cause short-stroking like your referring too. I can only imagine that if you see the gas blocks movement it is the cause of your problem. Bushmaster has decent customer service--call them, or replace the gas block yourself (pinned is better). I found the below resource helpful, when looking into a similar issue...

Source

 

[Alaskaman11]

Re: HELP HELP AR Trouble.

Good time to put a better gas block on. I disagree withe the notion of a pinned gas block. I see the Allen head as a better for adjusting the block, just use a level.

 

[TXTransplant]

Re: HELP HELP AR Trouble.

Yeah if the gas block moves at all, you are usually looking at immediate loss of function.

If the barrel is dimpled, the gas block shouldn't move, period. I'd send the barrel back to bushmaster to see what their solution is. Or you could send it to ADCO or someplace, have the barrel dimpled or have the gas block pinned, which is a sturdier solution.

Every gas block put in on every GI AR and M4 type rifle ever built (except I guess some of the original Mk12s) is pinned--you don't adjust a gas block's position once you've got it right.