"...What is an M15?..." Heavy barreled LMG version of the U.S. Rifle M-14 and a prohibited firearm just like the M-14. Nothing whatever to do with Armalite(who don't make an AR-15. That's a Colt TM, as I recall.) or the AR. mfdemon's fingers are too big for his key board, is all. snicker.
Anyway, how far an empty will go depends on the load/ammo used. I've seen 'em go anywhere up to 10 to 15 feet though. Then you need to consider the direction. Isn't always the same. Especially if the rifle isn't lubed correctly.