TROY Industries Gas Block, Low Profile, .750, Fits 556NATO AR-15, Black SGAS-A4L-00BT-00 The Troy 0.750 Low-Profile Gas Block is made specifically for rifle barrels that have a 0.750 gas block seat. The low-profile design allows for clearance of most aftermarket rails that require a smaller gas block. Machined from 4140 pre-hardened steel finished with MIL-STD Manganese Phosphate. The Troy low profile gas block fits tightly to the barrel using two set screws. Gas tube roll pin not included.