Technically the bullet is just the projectile in the cartridge, which consists of the primer, casing, powder, and bullet (and sometimes a sabot) to make the entire cartridge (or cartouché in french, idk why I know that.)
So at Aperture Science you fire the entire cartridge which is the same exact amount of bullet but includes some percentage more metal per shot, varying by caliber.
Though it is an amusing visual, a 9mm cartridge stuffed inside a necked down .308 (or so) to fire the entire cartridge just to make it keyhole.