A worm is a device used to remove unspent powder bag remnants from a cannon

Cannon

A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...

 or other piece of muzzle loading field artillery

Artillery

Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

. It usually took the form of a double corkscrew shaped piece of iron on the end of a long pole that could be twisted down the barrel to pick up any debris left over from the previous firing of the weapon.