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.