Ammunition with a muzzle velocity close to the speed of sound under many hunting conditions, or only very slightly below it. Subsonic rounds are favored for their perceived improved target accuracy, and for hunting.
Sam: These are .223 Caliber, subsonic rounds, the guy must've put a suppressor on the rifle.
Ava: Dude, who are you?
Sam: I ah...I just I...I just watch a lot of TJ Hooker.