Troy .936 low profile gas block is made specifically for bull barrel rifles that have a greater diameter than the standard m4/m16 .750 barrels. The Troy .936 low profile gas block works on both .223, 5.56 and .308 bull barrels (As well as other calibers). Finally there is an option for users who desire to replace their default gas blocks, or fixed front sight gas blocks to utilize a free float rail system that installs over the gas block.