This is a gas block with rails designed for an airsoft M4. This will fit a standard profile barrel with a .75" dia. I have not printed this yet so I'm not sure how it will come out.This should fit a standard profile M4 barrel. the holes on the sides are for M3 size set screws. Thread with a M3 tap and insert the set screws to keep it tight on the barrel. thee is a hole in the top to accept the gas tube if you have one.