When an attacking army lets loose a barrage of bullets all at once, it's called a volley.
A volley of bullets, arrows, or rocks describes a large number of them being shot or thrown simultaneously. Another meaning of the noun volley involves just one projectile: a returned tennis ball, usually one that hasn't hit the ground before being smacked by a racket. The military meaning of volley has been around since the 1500's, while the sports meaning didn't arise until the late 1800's. The root word is the Latin volare, "to fly."