AR15 short stroking issue

[Trent]

Cleaned an old eagle arms upper I had, took it to the range Friday. Ran fine for two mags, then started jamming up horribly and had to be retired for the day. When it jammed it was obvious that it was short stroking (based on the mangled empties I was pulling out of the receiver to clear it).

Disassembling the weapon revealed the action wasn't fouled. Gas port / tube was clear of obstructions.

But there was a LOT (and I mean THICK) amounts of fouling residue on the inside of the hand grips (it's an M4 carbine length gas system). It looks like a LOT of the gas is getting around the gas tube where it is pinned in the front sight post. Which could explain the short cycling (no other cause is evident - also, magazines and ammo were each conclusively proven good in another rifle).

I'm thinking it's time to fit and pin a new gas tube in?

Any other thoughts on what to check? Everything else in the upper seemed fine. Ejector, extractor, recoil spring, etc are all fine, chamber was clean, bolt travels freely, etc. The enormous amounts of gas residue in the handguard was the only oddity I found.

 

[10-96]

Are you sure it's coming from around the tube, or could it be coming from underneath the gas block? Both will give about the same results as you're describing. I know it isn't common, but I have a gas block that wasn't machined up to snuff.

Other than that- try the simple stuff first. Pull out your gas tube and see what carbon marks are on it- maybe that could give you some hints. Also, take a twisty tie and make sure you can run it through from the gas key to the bolt carrier.

 

[Trent]

Front end of the gas tube (about 1.5" worth of tube) was pitch black when I cleaned. Inside metal heat shield on the upper hand guard was pitch black with residue for the entire front half.

The port is definitely open to the barrel (when I run a pipe cleaner down the gas tube, I can see it come through the port in the barrel, when looking down the barrel).

I have another tube coming - if the problem repeats itself I'll swap out the entire font sight assembly (have lots of spares on hand, for those, bought @100 of them in a box off a retired FFL at a gun show for $10. Won't ever USE all of them but for $10.. they went home with me.)

 

[dprice3844444]

wondering whoever put it together missed the hole on the gas tube when they assembled it,or gas tube fracture

 

[Mac Attack]

I had the same thing happen to me. I had a burr in the barrel where the gas port hole was. I was shooting a lot more .22LR than 5.56 back then. The .22 LR rounds wore down the burr but not before the lead caused the port to be blocked. I had to pull the FSB, drill out the lead in the hole and then she was good to go.

 

[Trent]

I'm thinking gas tube fracture is most likely. But, I'm not ruling out gas eroding the aluminum front sight around the gas tube, or a gas tube that was bent during install preventing a good seal.

Rifle was LEO marked, produced during the ban, sold and marked LEO only by Cavalry Arms. Upper was manufactured by Eagle Arms, a division of Armalite.

Bought it off of a local cop the day the ban was lifted (was a "Trent's Victory Dance" purchase...), was very lightly used.

Replaced the thin profile barrel with a fluted chrome moly barrel in 08. I may have hosed up the fitting. I remember, specifically, that gas tube being a real bitch to fit in to the new front sight block on the new barrel.

 

[dprice3844444]

ya sure ya hit the pin hole on the tube when ya put it together?

 

[dprice3844444]

brownells makes a gas tube wrench.i use vicegrips myself,then tap the vice grip to install or remove

always use a steel gas block.

 

[Trent]

Will know when I pull it out, dprice.

Got a new tube on the way.