m15

My M4 has a S.I.R Rail on on it. Meaning that the barrel has Threads on top. I.E. Since I ordered a new Barrel i'm going to have to get it threaded and tapped. So that will be fun. I'm going from a 15" Barrel to a 8.7 " I've ordered a 220mm Inner barrel. New Muzzle I'm going to buy a lower...