Precision machined from 17-4 PH stainless steel, JKs booster piston is the component that screws onto the threaded pistol barrel, mating suppressor and pistol together. The pistons are available in 1/2×28 threads (nearly all 9mm and some other pistols) and .578×28 threads (.45 ACP and some other larger-than-9mm pistol calibers).