Shooting blanks out of your AR-15 is used primarily for the purpose of simulating live fire rounds. Especially in the military, blanks are used quite frequently.
Marines would use blanks in their weapons to simulate small scale battle scenarios. Doing this would simulate a live fire combat scenario, preparing soldiers and Marines for battle situations.
Blanks are also popularly used in reenactments (WW2, Civil War, any type of battle that there is a wide following). These men and women actors would use blanks in their guns to simulate the actual battle and firing of their weapons.
Firing blanks in your AR-15 is okay, and you need not worry about your weapon, it will not be damaged in the process, and the firing of blanks if commonly used for practicing purposes.
However, after you fire blanks in your weapon, you must clean it thoroughly. The nature of the blanks is rather corrosive, so if you don’t clean it, the residue could ruin your AR-15.
Another thing to note when attempting to fire blanks with your AR-15 is that you need an adapter that attaches to your weapon on the muzzle of the barrel. The adapter is necessary due to the fact that the blanks do not provide the same back pressure that live loaded rounds do.
If you attempted to fire blanks with your weapon, without the use of the adapter, the action will not cycle properly, and will not function the way it is intended to. A BFA, Blank Firing Adapter, is the name of the adapter used and attached to your weapon when firing blanks. The adapter closes off the end of the muzzle, and in so doing, it allows for the proper amount of back pressure to actually flow through the gas tube of your AR-15, and cycle the action.
It is also very important to realize, that your AR-15 packs quite a punch, and even though you are firing blanks, they are still be harmful to you as the shooter, and anyone in the immediate area.
When fired, the blanks can emit a large puff of smoke, that can be harmful to inhale, along with getting hit with certain types of blanks can actually cut you and cause bleeding.
But again, the most important thing to remember is to clean your weapon after shooting blanks. The round itself is, for one reason or another, much more dirty than a regular round, which is why the dust and debris emitted when shooting the blanks can be particularly corrosive to your weapon. The corrosion can cause very premature rusting, which is definitely something you want to avoid.
Shooting blanks through your AR-15 is not harmful, unless you aim at a person, don’t have the adapter attached for proper cycling, and finally, if you don’t clean your weapon after the use of blanks.