Among artillerymen, a nonexistent item used to trick rookies. It is commonly used to send rookies on a wild goose chase. (Muzzle blast is the burst of smoke and fire that erupts from the muzzle when a shell is fired.)
"Private, go to the supply sergeant and see if he can find you a bucket of muzzle blast."
It's when you carelessly aim your gun in someone's direction without intentionally meaning to do so. Newbie gun owners are often at fault of this, but also when people unholster a firearm they can make the same mistake unknowingly. Muzzle sweeping is violating the second rule of firearm safety.
T-money whipped out his nine from being strapped in between his belt and boxers and muzzle sweeped us all as we ducked anxiously in fear.