For any AR-15 / M16 with a removeable (not pinned) military style birdcage flash suppressor, to make an internal (hidden) field expedient blank adapter:
1. Take a piece of fired 5.56 brass and punch out the fired primer. If you are a reloader, simply start with a new piece of un-primed brass.
2. Place the now de-primed brass into a vice (or hold with pliers). Position the base of the cartridge case protruding horizontally (i.e., neck of cartridge and about 2/3 of cartridge length securely pinched in the vise/plier jaws).
3. Take a small hacksaw blade and saw off the base of the cartridge case to within about 1/8 to 1/4 inch of the solid cartridge base (where the primer pocket resides). Saw it evenly and straight across. You may have to try this a couple of times until you get the right length. Have an extra de-primed brass case or two...
4. You will now have a small brass "button" with a hole in the center.
5. Remove (unscrew) your weapon's birdcage from the threaded end of the muzzle.
6. Pace the brass button (eyeball centered and cartridge base down) onto the crown of the muzzle (pointing muzzle vertically). Re-attach the flashhider by screwing it down hand tight over the button. The birdcage will completely cover and hide the brass adapter. If you stare down the muzzle, you'll see a brass circle blocking the bore with a primer-diameter hole in the center.
7. Your AR is now equipped with an internal blank adapter which will allow it to function & cycle on semi-auto (or full auto if you have such an option). When fired, gas from the blank cartridge will rush down the barrel and encounter your internal blank adapter. The open primer hole of the adapter will allow just enough gas pressure to bleed off to simulate the dwell time of a normal bullet in the barrel. Enough gas will go to the direct impingement gas tube to reliably cycle the action and load another blank.
8. Fire until you are bored or out of blanks. If the weapon cycles sluggishly, you probably need to tighten down the bird cage a quarter turn or so.
9. AFTER USE... YOU MUST REMOVE THE DEVICE AND VISUALLY CLEAR THE BORE. If you forget (easy to do) or allow someone else to access the weapon (they won't see or know about an internal blank firing device)...YOU WILL HAVE A BARREL BURSTING KABOOM THE NEXT TIME YOU FIRE A LIVE ROUND.
10. ONE OF THE REASONS MILITARY BLANK ADAPTERS ARE USUALLY OVERSIZED AND BRIGHTLY COLORED IS TO REMIND THE OPERATOR THAT HE/SHE MUST REMOVE THE DEVICE BEFORE USING BALL AMMO.
11. This old Soldier's trick has seen decades of use by GIs needing an improvised (and louder) blank adapter. It is especially useful for graveside ceremony or memorial service firing parties...as the sound is louder than when using an issue blank adapter (which sounds tinny on video). Additionally, the weapon looks stock and more photogenic (firing party photo looks like real rounds being fired).
Figure The Odds...