Test fired a friend's 11.5" last night and he was having problems with the cycling. The bolt won't fully move back to chamber a new round, its not an extraction problem as it will extract the empty shell but the bolt will get stuck halfway along the magazine feedlips. Not a magazine problem as well as we tested it with good mags and ammo.  

Barrel used was a J&T (shaw) 11.5", we assembled it with a fixed front sight, gastube. I noticed the front sight canted slightly to the left but the gunsmith insisted it was ok, so the problem. So I said it could be because the gasports weren't properly aligned. We eventually changed the barrel to a newer one same J&T 11.5", but this time the fixed front sight came factory assembled. Still the same problem. Btw it has a muzzle brake on it.  The started as a 14.5" upper that ran fine but my friend wanted to change it to a shorter barrel. What could be causing this? Is this a barrel problem?

I have a LMT 10.5 upper assy that I use, zero malfunctions.

Quoted: Quoted: The started as a 14.5" upper that ran fine but my friend wanted to change it to a shorter barrel.   Open the gas port to 85

Someone here has a really nice gas port diameter list.  Maybe they'll see this.

.085"

What ammo are you using? And check to make sure the carrier key isnt loose.

Make sure that the gas port hole of the barrel is aligned to the front sight gas hole.  If that is not aligned then there is your problem before making the hole bigger.

Quoted: Test fired a friend's 11.5" last night and he was having problems with the cycling. The bolt won't fully move back to chamber a new round, its not an extraction problem as it will extract the empty shell but the bolt will get stuck halfway along the magazine feedlips. Not a magazine problem as well as we tested it with good mags and ammo.  ...

Wait a sec...the bolt get's STUCK? Something's up then. Even if it doesn't cycle, the bolt should still come back half way, THEN slam back forward due to the buffer spring pressure.

Improper hammer and bolt carrier would be my guess.
Carrier getting snagged on the hammer as the bolt moves forward.
I had the same problem.
But what do I know,
I'm just a dumb KanukAG.

what the others have said and im gonna throw in= crushed upper reciever.



had one that was crushed slightly and would drag the bolt carrier.

I initially though it could be the bolt or receiver but using the same upper and lower we changed it back to its original configuration with a 14.5" barrel and it ran fine.

Shouldn't the barrel come with the proper gas port diameter?

From the looks of it its not getting enough gas to move the bolt fully rearward, even if the front sight was properly aligned to the barrel etc. Maybe I need to open up the gasport more.

did he cut the bbl down or just swapped the bbl?

Quoted: Test fired a friend's 11.5" last night and he was having problems with the cycling. The bolt won't fully move back to chamber a new round, SHORT STROKING: May be caused by not enough gas going thru the gas tube. its not an extraction problem as it will extract the empty shell but the bolt will get stuck halfway along the magazine feedlips. WEAK BUFFER SPRING: The buffer spring may be too weak to push the bolt back into battery and strip a new round from your magazine. Did you change the buffer spring to an aftermarket one sold around here? Not a magazine problem as well as we tested it with good mags and ammo.   Barrel used was a J&T (shaw) 11.5", we assembled it with a fixed front sight, gastube. Is the gastube new? Free of obstruction / fouling? I noticed the front sight canted slightly to the left but the gunsmith insisted it was ok, so the problem. So I said it could be because the gasports weren't properly aligned. We eventually changed the barrel to a newer one same J&T 11.5", but this time the fixed front sight came factory assembled. Still the same problem. Again the same question: Was the gas tube new? Does it fit snugly in the FSB = no leaks? Btw it has a muzzle brake on it.   Will not affect the cycling. The started as a 14.5" upper that ran fine but my friend wanted to change it to a shorter barrel. What could be causing this? Is this a barrel problem? I have a LMT 10.5 upper assy that I use, zero malfunctions.

Hey Eyxxx. How's your friends AR running?

I listed a couple of things up there for you guys to consider. Obviously your friends SBR is short stroking. But what is causing it is the problem at hand. From the symptoms you mentioned, the logical area of concern is the gas port or gas tube. But when you mentioned that it gets stuck midway and does not strip a round, I immediately thought of a buffer spring problem. I had the same problem with aftermarket crap sold here that was supposed to reduce cyclic rate. Got rid of that spring ASAP. So check that out.

I believe it's a gas problem though parts used were new. We decided to scrap the whole thing and return it to its original configuration of 14.5" and build a totally new upper using better parts and have him return the crap parts he got from the dealer.

I have a Colt heavy barrel 11.5 and I asked this same question. A reputable member here told me for a heavy barrel 11.5 that it should be between .086 and .094

I made mine .089 and WOOHOO!!! Flawless function single and full-auto.