These gas blocks along with Aero Precision's gas tubes create an efficient and reliable gas system for your AR15 or AR 308 rifle build. Features: Fits barrels with .875 Outer Diameter Constructed from 4140 hardened steel Nitride coating Low profile for easy fitment Set Screws are Steel 10-32 x 3/16 Knurled Cup Point Made in the USA *Roll pin not included* Please note: We do not recommend the use of red thread locker on the threads of the set screws. If you decide to use thread locker we recommend using the non permanent variety.