Aluminum .875″ gas block with top picatinny rail. This gas block has the gas port off set .025″ rearward. This allows the gas port in the block to line up with the port in the barrel when a hand guard end cap is installed. The end cap is typically used to retain the front end of standard GI plastic hand guards. The rail on the top of the block is at gas block height, NOT same-plane with the receiver.