Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
When purchasing a new barrel for an AR-15 style rifle, you may wish to fit a low profile gas block. Many gas blocks are located on the barrel by set screws and a dimple on the bottom of the barrel directly opposite the gas port. Tighten the set screw into the dimple and the hole in the top of the gas block where the gas tube resides should center itself over the gas port.
At times, the builder may be confronted with a barrel with no dimples. How do we locate one? A simple fixture marketed by Black Rifle Disease Engineering and Brownell's (part #130-006-130) makes this job relatively easy. The fixture consists of a collar and two set screws.
Slide the collar onto the barrel (.750" diameter) and locate the rear (pointed) set screw over the gas port. Tightening the screw into the gas port should locate the fixture. Tighten the forward screw to snug everything up. Two holes are located in the bottom of the fixture. These are drill guides for an 11/64" drill.
Clamp the fixture and barrel in a drill press vise and level it. Dimple the barrel through the rear hole. Hole spacing in the fixture appears to be about .456" and gas block hole patterns vary but you can install your gas block, tighten the rear screw in the dimple and drill another dimple carefully through the forward hole in the block.
Degrease the screw holes, install the gas tube and slide the gas block assembly onto the barrel, inserting the end of the gas tube through the barrel nut.
Apply thread locker to the set screws and tighten them into the barrel dimples. Free-float fore-ends should be installed prior to installation of the gas block in most cases.
Black Rifle Disease
Here the Black Rifle Disease Engineering fixture is installed on barrel. A pointed set screw is tightened into the gas port to locate the fixture.
Dimple the barrel
Level the barrel in the vise and use an 11/64'' drill to dimple the barrel. The fixture will ensure it's located in the right position.