AR15 SBR Short stroking, help needed.

[LavaRed]

As some of you may remember, I recently completed 2 L119A1 builds, both select fire, one standard length, one with a short GSE buffer, 10.5" barrel, and a Paladin adjustable gas block
Well, testing the shorty one, I have encountered lots of problems. Specifically:
The rifle will fire about 3 rounds in semi, or 2 in full, before jamming. Usually, the chamber will be empty, and the hammer uncocked, tho ocasionally there was a round chambered with the hammer uncocked.
I noticed the gas tube backing out of the gas block. It's held to the gas block by a couple of tiny allen screws. I dimpled the gas block with a diamond ball file, but it continued to back off after 3 shots or so, deforming the dimples.
Replacing the GSE tube with a regular skeleton stock and commercial 3oz. buffer yielded the exact same results.

So, I need your help correcting this problem, as I am not very familiar with the AR platform, and I'm at a loss as to how to correct the problem.
Thanks,
Lava

[mr fixit]

Not an expert by any means, but it sounds like not enough gas. If there is not enough gas to fully retract the bolt, you end up with an empty chamber. I could see the possibility of just enough gas to extract a round and barely chamber another with out getting the hammer fully cocked.

If it were not for the hammer being un cocked, i would strongly suspect a magazine issue. With the hammer not being fully cocked, i suspect it is gas related.

possible problems to look for:
leak in gas tube
leak in gas block
gas block not open enough
too heavy buffer
too heavy buffer spring
carrier key loose

my first suggestion is to replace the adjustable gas block with a standard gas block.

[mr fixit]

Did you get your problem resolved?
If not, you might get more suggestions if you re-title your question asking for help for Ar-15 rather than L119A1.

[CMV]

The gas tube should be secured into the gas block with a roll pin. The gas block itself should secure to the barrel with set screws, clamping screws, or taper pins depending on type. Regardless, neither should move. If the gas tube is separating from the gas block, that's my guess to the problem - the hole in the gas tube isn't staying aligned with the hole in the barrel. But that's a symptom of [what I suspect] a missing roll pin.

[gunny50]

The gas tube should be secured into the gas block with a roll pin. The gas block itself should secure to the barrel with set screws, clamping screws, or taper pins depending on type. Regardless, neither should move. If the gas tube is separating from the gas block, that's my guess to the problem - the hole in the gas tube isn't staying aligned with the hole in the barrel. But that's a symptom of [what I suspect] a missing roll pin.

As far as I recall the Paladin adjustable gas block does not have a roll pin option for the gastube

Gunny

[CMV]

It may not - I don't have one. Just going by the ones I use & what's "typically" found on an AR.






If it uses set screws to hold the gas tube, maybe a knurled tip set screw would work?

[gunny50]

double post.
Gunny

[gunny50]

It may not - I don't have one. Just going by the ones I use & what's "typically" found on an AR.
If it uses set screws to hold the gas tube, maybe a knurled tip set screw would work?

Using the paladin the gas tube is held in place by the 2 stews on top or as some have them one at each side.
The Paladin can not use a roll-pin as the gas selector is in the away.
Its the way this block was designed.

You could modify it in a way as the Armalite SASS gas block ( use hydraulic, brake connectors to hold the tube in.



Gunny

[LavaRed]

Thanks for the ideas folks.

Unfortunately, I replaced the Paladin with a regular pinned gas block altogether, and the problem still persists.

I ended up ordering McFarland one piece gas rings, and sent the barrel off to the machinist to have the gas port enlarged from 0.73" to 0.93, which is the maximum allowed under Colt specs aparently. I will also be replacing the cam pin, just in case, and the gas buster charging handle with a regular part.
Can't do much until the parts arrive, however.

[redtazdog]

What buffer are you using ?
The heavier H2 buffer will hold the bolt closed longer and allow the brass to shrink back down in size
so it will extract easier, faster and stronger.
A buffer that too light will allow the bolt to unlock sooner when the pressure in the brass is still high
causing the brass to be too tight in the chamber slowing the extraction and giving the bcg a short stroke.

[CMV]

Thanks for the ideas folks.

Unfortunately, I replaced the Paladin with a regular pinned gas block altogether, and the problem still persists.

I ended up ordering McFarland one piece gas rings, and sent the barrel off to the machinist to have the gas port enlarged from 0.73" to 0.93, which is the maximum allowed under Colt specs aparently. I will also be replacing the cam pin, just in case, and the gas buster charging handle with a regular part.
Can't do much until the parts arrive, however.

The short-stroking or the gas tube separating from the gas block?

[LavaRed]

What buffer are you using ?
The heavier H2 buffer will hold the bolt closed longer and allow the brass to shrink back down in size
so it will extract easier, faster and stronger.
A buffer that too light will allow the bolt to unlock sooner when the pressure in the brass is still high
causing the brass to be too tight in the chamber slowing the extraction and giving the bcg a short stroke.

At first I was using the GSE short pistol buffer tube, which exerts far more force than an H2 buffer. First I used it with a pure tungsten weight that I made myself, then with the standard steel weight that came with it. When that failed, I put on a skeleton stock, and tried that first with an H2 buffer that I borrowed from another rifle, then with a commercial 3 Oz. The only change was when I replaced my tungsten weight with the steel weight on the GSE tube: The hammer started getting cocked most of the time, where before it was mostly uncocked after firing.

[LavaRed]

Thanks for the ideas folks.

Unfortunately, I replaced the Paladin with a regular pinned gas block altogether, and the problem still persists.

I ended up ordering McFarland one piece gas rings, and sent the barrel off to the machinist to have the gas port enlarged from 0.73" to 0.93, which is the maximum allowed under Colt specs aparently. I will also be replacing the cam pin, just in case, and the gas buster charging handle with a regular part.
Can't do much until the parts arrive, however.

The short-stroking or the gas tube separating from the gas block?

The short stroking. The tube separation was corrected with the standard gas block.

[CMV]

If you said what ammo you're using I missed it. If it's Wolf, that's typically loaded on the lighter side and extracting the steel cases might be putting extra drag on the BCG. If it's standard commercial .223, maybe try some MILSURP 5.56mm ammo. 5.56 is generally loaded a little hotter than most commercial .223. Not a huge difference, but maybe enough to get you running until you get it gassed right.

Another thing to try if it's pretty close, take some of the preload off the hammer spring. It will resist the bolt carrier less when cycling. If hammer springs are as cheap & plentiful there as here, the easiest way is to just cut an entire leg off the spring. Just make sure it's not the leg that catches on the trigger pin to hold it in place (or flip the pin if it is). 9/10 times, an AR will still ignite a hard primer like a CCI #41 or Wolf SRM with one entire leg chopped from the [milspec] hammer spring and a very small bend to take some preload off the remaining leg. That's almost an insignificant amount of drag relieved from the bolt carrier, but sounds like you're really close to having it cycle so maybe either or both of these suggestions will make it run. Doesn't cost anything to try.

[LavaRed]

I'm running Guatemalan milsurp M193 pattern ammo. No, AR parts are almost nonexistent here. But, I will order a spare spring and try this anyways.