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.
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.
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?
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.
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.
Generally speaking, there should be a slight gap between the gas block and the shoulder of the barrel, for the ports to line up correctly. The gap represents where the handguard cap would have been if you were not running a free float. (Some manufacturers may do it differently, and I am not sure exactly how LT does it).
I can’t be sure from the picture, but it looks like the gas block may be sitting a bit too far forward - if so, that would shut down (or greatly reduce) the gas system.
Remove the handguard and reposition the block. Be sure to was red loctite on the set screws. I’ve built several rifles using LaRue gas blocks, none have sit that far forward.
ETA: I reread your first post. That block is not back far enough. There should be a gap of 3/32" to 1/8" tops. Right now you are cutting off your gas.
I want to add that the D&H mags are dated 11-05. I remember reading a thread on the “other” forum about people having problems with that date.
Does anyone know if that was true?
Did you only try this once? Or did you test this multiple times?
How did it extract when you ran this test? When you were having the problems the other day was the empty case still in the upper or was it still in the chamber?
Did the FTe look like it was trying to chamber a new round while the spent case was in the way?
I would have to concure that the gas bolck is the issue. However, if the weapon is cycling and locking back onthe one round test, that could mean its something else. Not sure what at the moment though…
You said you are shooting Q3131 in it now. Has there ever been any Wolf shot through it? Reason I ask is, I have had to work ona couple of AR’s at work recently that the owners had been shooting Wolf through and then switched to brass cased ammo. The Wolf would run fine, but the brass cased rounds would stick in the chamber and fail to extract after just a few rounds were through the gun and the chamber heated up.
