Just go to canadian tire and buy a set of mini/micro hex drivers. Or you can get better ones with hardened steel (so you don't strip the screw) at RC hobby stores. Don't get the L shaped allen keys, I find them a pita to use, get a set of proper drivers.
Hex screws down to 0.7 have been used in RC for years. I think the ones used as set screws for those flash hiders are either 1.0 or 1.5.