The Presma low profile gas block fits AR barrels with a .750" gas journal. It sits low enough to clear free-float and standard handguards, so you can run a slim rail over it without cutting or spacing anything. The block is machined from stainless steel, which stands up to the heat and carbon at the gas port. It is held on with set screws, so you can install it without pinning the barrel. That also makes it easy to remove later if you change handguards or barrels. Features: Low profile AR gas block Fits .750" gas journal barrels only Stainless steel construction Set-screw retention Sits under free-float and standard handguards Confirm your barrel has a .750" gas journal before ordering. This block does not fit .625", .875" or .936" journals. When you install it, line the block's port up with the barrel's gas port before you tighten the set screws, and use thread locker. Function-check that the bolt locks back on an empty magazine before you trust the setup.