I have one of the red M16 blank firing adapters shown above. It does have a hole down the center of the shaft, just as NavyLT says, with two ports out its sides just before where the shaft is threaded. The ports are about even with the edge of the red tab at the top of the picture.
NRA Certified Instructor: Pistol, Rifle, Shotgun, Home Safety, Personal Protection, Range Safety Officer
NRA Life Member