Need help diagnosing a problem with my rifle...

[RHR]

I took my new upper out for the first time today. Shot 60 rounds 5 rounds at a time just to get a feel for it and everything was fine. I loaded 20 rounds in a 30 round mag and that's when the problem started. It would fire one round and the bolt would get stuck open about 1/4" and it wouldn't extract. I had a hard time pulling the charging handle back to get it to extract. I fired 40 rounds like this and quit. I took the bolt apart and didn't see anything wrong with it or the extractor.

The rifle is a 14.5" LMT M4 upper with LaRue 9.0 and gasblock. I'm running a LMT standard M16 BCG with Crane O ring and a PRI Gas Buster charging handle. The lower is an LMT with 2 stage trigger and stock carbine spring/buffer. The mags were 2 D&H black teflon with gray Magpul followers and 1 Labelle gray teflon with brown Magpul follower.

Any ideas? Thanks!

[BravoCompanyUSA]

I took my new upper out for the first time today. Shot 60 rounds 5 rounds at a time just to get a feel for it and everything was fine. I loaded 20 rounds in a 30 round mag and that's when the problem started. It would fire one round and the bolt would get stuck open about 1/4" and it wouldn't extract. I had a hard time pulling the charging handle back to get it to extract. I fired 40 rounds like this and quit. I took the bolt apart and didn't see anything wrong with it or the extractor.

The rifle is a 14.5" LMT M4 upper with LaRue 9.0 and gasblock. I'm running a LMT standard M16 BCG with Crane O ring and a PRI Gas Buster charging handle. The lower is an LMT with 2 stage trigger and stock carbine spring/buffer. The mags were 2 D&H black teflon with gray Magpul followers and 1 Labelle gray teflon with brown Magpul follower.

Any ideas? Thanks!

Basically it ran ok for a short bit, and then it pretty much stopped cycling.
Assuming the bolt carrier group can travel freely in the upper and works to hand cycle and eject rounds...
My first guess, would be to look at the gas system.
If your firing weapon and the bolt does not move to the rear, you may have lost most/all pressure. Take a peak at the gas block to insure it did not come loose or rotate well off center. Also look at the gas tube, to insure the roll pin is securing it.
I would also look closely at the gas key on the carrier - however I have never personally seen a LMT carrier gas key get loose.
Contact the company that set up the upper for you, it should be an easy fix.

[C4IGrant]

Paul gave you some good ideas. I would make sure that the bolt and carrier is properly lubed and try some different mags and ammo.

I would load one round into a mag and see if there is enough pressure to lock the bolt back.



C4

[Boom]

The guys above covered it. Check gas system and change mags. See if problem returns.

[RHR]

Thanks for the replies. The ammo is Winchester Q3131.

[C4IGrant]

Thanks for the replies. The ammo is Winchester Q3131.

Could be a bad batch (not likely though).


C4

[RHR]

I just looked at my rifle again. The gasblock doesn't seem to be loose. It does look like there is an 1/8" gap between the gasblock and the step up in the barrel. Is this normal?

[M4arc]

Is the gas block pinned on or bolted on? If it's bolted on I suspect it could have moved while still being somewhat tight.

Do like Grant said and fire a round and see if the bolt locks back. At least then we'd have a good starting point but right now the gas block would be my #1 suspect.

[RHR]

M4arc, I'm not sure about the gasblock. I'll check with the person that built my upper.

[K.L. Davis]

RHR

I just noticed your post... sounds like the advice you have gotten is good.

A lot of folks do not realize that the gas block (and everything else) tries to move forward under the recoil of the rifle, it only takes a little bit to block off enough of the gas port enough to effect the function of the rifle.

Rifles that use a set screw mounted gas block *should* have the barrel machined 180 degrees opposite of the gas port, for the rear set screw to recess into -- this ensures alignment and helps prevent movement of the gas block.