Just have them put a .300" freebore on your reamer. Thats what you want for a 300 Hybrid. It wont fit in a mag, you will need to do a 338 Norma imp to fit in a mag. You can run a 1 in 10 but most of the ones I do are 1 in 9 or 1 in 9.35 for the elr stuff. A little extra spin helps in the transonic.