There is usually a notch in the barrel where a a roll pin indexes and secures the front sight and/or gas block in place. Some gas blocks use a set screw. All work equally well. Replacing the gas block isn't really hard to do. The issue with the R-15 is that it's camo dipped, while any replacement railed gas block will probably be black. If you don't mind this, then your options are wide open as the R-15 uses the same parts as any other AR-15 (it's a rebranded Bushmaster).

Mike


Mike