The Geissele Super Gas Block (SGB) is precision investment cast from 17-4 precipitation hardening stainless steel, which combines superior corrosion resistance with high strength. It is designed to fit under most full length free floating rails such as the Geissele Super Modular Rail. The bore of the SGB is machined to closely fit around .750" diameter mounting bosses found on most M4 Carbines, AR-15 and AR-10 type rifles.