AR15 Short cycle

[joem]

A friend of a friend built a AR15. Young man that I doubt has any gun building experience. He reports that after one mag it short cycles. After a cooling period it runs fine for one mag. My friend called and asked what might cause this and things to check. Told him to Check usual stuff, very dirty, loose gas block, broken gas rings, check buffer tube assembly. I'm wondering if he bought a upper and barrel seperate and assembled it himself. If that doesn't work, any suggestions are welcome.

[Jim in Salt Lake]

Was the gas block lined up with the gas hole in the barrel when it was installed? If it's a clamp on style and not square over the hole in the barrel, even if tight can cause this problem.

[joem]

I don't know for sure. If my friend can't figure it out he will bring it over for me to look at. I would bet a gas block problem.

[Litt'le Lee]

shoots for 30 rds

and it quits?? sounds like a new rifle and NO LUBE__shoot it WET WET WET

[joem]

It was reported that when it cools off it will function until hot. That's all I know at this point.

[Hecklerusp45]

No expert here, but when it gets hot something is expanding and causing a bind?

[S.B.]

I ran into this very problem with a recent build(with Russian junk steel cased ammo), I changed both the buffer and recoil spring to Colt products and ran fine.
Steve

[joem]

My friend found that the person that did the build left out one screw in the gas block. Must have allowed the gas block to move a bit. Young man doing his first build.