There will (usually) be something called a Blank Firing Adapter used, which is really nothing more than a small plug which blocks the barrel enough for the gasses from the blank to build up, and provide enough pressure to properly cycle the firearm.

These work well because they don't really require modification of the rifle itself, and are usually painted bright red to aid in identification. Unfortunately that makes them kinda useless for filming purposes.

Yeah, you are correct, this is what we used, and during filming that wouldn't look good yeah. I remembered these more like a safety precaution - although sometimes some managed to fire off one of those. Probably we were talked about that how they help reloading for blanks, but I didn't remember.

Then there were also machine guns we fired blanks with, I don't remember if we fired live ammo also from machine guns or whether they also had nozzles. Actually, never mind, I think we did also do target practice with machine guns.