The hand mortar is a firearm and early predecessor of modern grenade launchers that was used in the late 17th century and 18th century to throw fused grenades. The action was similar to a flintlock, matchlock, or wheellock firearm (depending on the date of production), but the barrel was short, usually 2 inches (5 cm) to 4 inches (10 cm) long (though some are reported to have barrels up to 13 inches (33 cm) long), and had a large bore to accommodate the grenade; usually between 2 and 2.5 inches (5 to 6 cm).
You can use a weapon that has the Ammunition property to make a Weapon Ranged Attack Roll only if you have Ammunition to fire from the weapon. Each time you Attack with the weapon, you expend one piece of Ammunition. Drawing the Ammunition from a Quiver, case, or other container is part of the Attack (you need a free hand to load a one-handed weapon).