I just recently finished a new ar that I pieced together. It has a dpms lower, bushmaster factory built upper, and an unknown brand of bolt carrier. First time out I got 9 rounds thru it before it siezed up and needed to be taken apart with a brass hammer! Found out one of the gas rings was broken and jamed in between the bolt and the carrier. So I figured maybe it was just a bad ring. Replaced it and took it out shooting again. 7 rounds before the exact same failure! Any Ideas?? Maybe time for a name brand bolt carrier?

Try checking the carrier bore for burrs, especially at the gas key hole.