A Sub-Machine Gun is a fully automatic, pistol caliber weapon with a high rate of fire. In the Resident Evil series, Sub-machine guns are classed under Machine gun.
Like the similar Assault Rifle, the sub-machine gun is recommended for use against tightly grouped enemies and for harassing larger B.O.W.s.
Most sub-machine guns use pistol ammunition, similar to handguns. To avoid same ammo usage problems in some games, SMG's use custom pistol ammo, different from handgun bullets; the exceptions are the Submachine gun of the Outbreak series and the Ammo Box 50 in Resident Evil 6, which uses the same 9mm ammo of handguns.
In most iterations, they generally lack firepower, stopping power, and general accuracy, even moreso than handguns in general. These drawbacks however are compensated by their larger magazine capacity, very rapid fire rate, and ammo abundance (in cases where they use different ammo types than that of the handgun). Unique to older generation titles, fully-automatic weapons such as submachine gun and machine guns all have their ammunition count as a percentage instead of numbers per magazine.