The Luth-AR 24″ Fluted Bull Barrel Complete Upper Barrel Assembly Starts with a 24″, .223 caliber fluted bull barrel made from 416 SST with a 1-9″ twist mated to a A3 Flattop receiver which includes a bolt carrier, charging handle, forward assist and ejection port cover. The upper is then topped off with a lo-profile gas system and an ergonomic 15″ Palm Handguard with KeyMod slots and a Picatinny top-rail.