Foxx, it should be a 16" twist. That was the fastest readily available blank that Jim has a reliable access to and it is plenty fast enough for the 450BM velocities.

emtrescue6, I have mine in a Tupperware stock (since its cheap, light, and serves great as the truck/ATV gun I intended this for) and recoil is about the same as my 12FV 308, or like a sporter 243. I had Jim thread the muzzle for a brake, and I put one on it I built, but haven't tried it yet.