PRODUCT DESCRIPTIONRSTA muzzle adapters mount our over the barrel, RSTA and SPR sound suppressors. Choose from our high efficiency 1/2×28 (5.56mm), and 5/8×24 (7.62mm) flash suppressors. NOTE: these mount adaptors require a 2.55″ of barrel rear of the muzzle crown or 1.93″rear of the thread shoulder to be no larger than .775″ outside diameter. This is the barrel contour for Armalite AR-10 rifles. RSTA muzzle adaptors are CNC machined from solid 17-4 precipitation hardened stainless steel for maximum heat and wear resistance.