Wikipedia states that: A "round" is a term synonymous with a fully loaded cartridge containing a projectile, propellant, primer and casing.
And: .A cartridge, is a single unit of ammunition. For a modern small arms cartridge this is the combination of bullet, propellant, primer and cartridge case in a single unit.
Dictionary.com:
(Round)
a charge of ammunition for a single shot.
(Cartridge)
1.
Also called cartouche. a cylindrical case of pasteboard, metal, or the like, for holding a complete charge of powder, and often also the bullet or the shot for a rifle, machine gun, or other small arm.
2.
a case containing any explosive charge, as for blasting.
Round also makes more sense, as in "Round in the chamber".
I'm getting confused now.