Ammunition design specifically for Gravity Guns. They are usually referred to as IMMs.
While most gravity guns use rocks or shrapnel for ammunition, an Infinite Mass Missile was designed to be used by a gravity gun. The missile carries a standard Space Age Warhead, but the Artificial mass block makes it shoot out the barrel at the speed of a bullet. These missiles can do devastating damage to a ship, not only do they explode like a normal missile, but the shrapnel is also moving through the ship, tearing through light armor plating and interior walls like butter due to the high velocity they are moving at.
They look like typical missiles.