M224 Mortar

A mortar is an indirect fire weapon that fires shells at low velocities, short ranges, and high-arcing ballistic trajectories. It is typically muzzle-loading and has a barrel length less than 15 times its caliber.

A mortar is relatively simple and easy to operate. A modern mortar consists of a tube into which gunners drop a shell, which is usually referred to as a bomb or round. The tube is generally set at between 45 and 85 degrees angle to the ground, with the higher angle giving shorter firing distances. The shell contains a quantity of propellant. When it reaches the base of the tube it hits a firing pin, which detonates the propellant and fires the shell. Some larger caliber mortars have a string operated firing pin instead of a fixed one.

Battlefield 3
While the exact mortar type is unknown it is speculated to be the M224 60mm Mortar or possibly some other variants.