"The FMD, a very popular personal conversion kit, enhances the fusion mortar giving it a faster projectile that deals more damage, but has a smaller explosion radius." In-Game Description.
The Fusion Mortar Deluxe is a variant of the Fusion Mortar that deals greater damage on a direct hit but at the cost of AoE. Like the standard Mortar it is a long-range artillery weapon that fires a powerful ballistic projectile. When the round hits the ground in less than 2 seconds, it lingers for a moment before detonating. When used long-range it will explode on contact with any surface. The FMD has the greatest damage of any weapon in the game, dealing a maximum of 1400 damage.
When attacked by a FMD-wielding Juggernaut, look for the highly-visble contrails of the mortar projectile, making it easy to dodge even for a heavy class, despite the faster speed of the projectile. When indoors always move towards the Juggernaut in order to restrict space between you and him, heavily reducing the effectiveness of the FMD lest he damage himself.