I was looking for semi-custom a while back myself. I shoot 50-55 grn bullets just about exclusively. My intentions for this upper was dogtown, target, & maybe some predator hunting. The builder offered 1/8 & 1/12. After talking with him about it I went 1/12 & a 'wylde' chamber, or version thereof. He said I would be fine up to 60grns/62? in the 1-12. I really can't see myself going above 55 though. I also went 22", .75" fluted barrel. I have other 1/9 ARs in various configurations for other stuff. If I planned to carry it a lot I would of went shorter & lighter.
