I'm calling BS on this one. No such a thing.
Blank cartridges are frequently used in the military to allow realistic training exercises.
Rifles such as AR-15s and M-14s are gas-operated semi-automatic. The expanding gases which propel the bullet down the barrel are used to retract the bolt, eject the used shell casing, and load another live cartridge into the chamber, making the rifle ready to fire with another trigger pull.
These rifles have a small hole near the muzzle which directs gas into the space in the rifle where the expanding gases create the force to actuate the bolt.
In an M-14, and probably an AR-15 as well, the pressure created by a normal cartridge is sufficient to operate the mechanism, but the pressure provided by a BLANK cartridge is often not enough (because the gas starts venting from the muzzle immediately and less powder is used in blank ammo) and the rifle will tend not to cycle properly when firing blanks.
The solution that I am familiar with was used with an M-14 and was called a "blank-firing adapter". This was a device which attached to the muzzle and blocked much of the gas from venting so that the pressure would be higher when firing blanks and the rifle would cycle properly.
A serious problem would develop if a soldier neglected to remove the adapter and attempted to fire real ammunition, probably resulting in destruction of the rifle and great potential for injury.
For this reason, the adapters are made somewhat larger than necessary to do the job and they are painted bright red.