A Definition of the Term "Round" in Firearm Shooting

Smoking Bullets Falling on Ground
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Definition

In firearm shooting sports, the term "round" refers to a single piece of ammunition. This may mean a single metallic cartridge with its casing and bullet, a shotgun shell casing and its pellets or slug, or a muzzleloader powder load and projectile. The term should not be confused with the bullet, which is the metal projectile that is accelerating off the metal cartridge casing, through the gun barrel and downrange when the trigger is pulled.

The bullet itself actually ceases to be "round" as soon as it accelerates down the gun barrel. 

In sport skeet shooting, the term "round" may also refer to a 25-shot session of shooting.

Origins of the Term

The genesis of the term round is likely because almost all ammunition cartridges and shotgun shell casings are cylindrical in shape, and thus can be called round. No other plausible explanation has been offered.