Mounted Machine Guns can be utilised by all the enemies in Army of Two and Army of Two: The 40th Day. These heavy weapons spray a deadly hail of high-velocity bullets, and have a virtually limitless supply of ammunition. They have heavy frontal armour and are sometimes dug into defensive positions.
The easiest way to neutralize an MMG is to focus its attention on the player with the highest aggro, allowing the other player flanks it and stealthily kills the MMG's occupant. It is inadvisable to rush an MMG, as they can do a tremendous amount of damage in a short space of time.
A second way to kill them is to get close and throw a grenade near the side or just behind the gunner. You can also use projectile explosives. But make sure you don't shoot the shield directly or there will be almost no affect.