The Multigunner IFV is a quick but weak unit that can change it's weapon according to the unit inside it. It's default weapon is a rocket launcher that can be both anti-air and anti-personnel. Only the Allies have this unit, and can be trained at the armour factory.
Attack Dogs
Starting with attack dogs, once you garrison dogs into these vehicles, they in turn change thier weapons into loudspeakers that can amplify the dogs bark and stun/paralyze the enemy unit when the vehicle is ordered to attack.
Peacekeepers
When the Peacekeeper enters the vehicle, the vehicle "grows" shotguns that are only anti-personnel.
Javelins/Rocket Soilders
When Javelins enter the weapon stays the same, so no need to put jav's into the vehicle.
Spies/Engineers
Spies and Engineers can enter the Multigunner too. When a spy enters the vehicle, the weapon changes to a sniper, take note that the sniper can only be used on infantry. The engineer's "weapon" when he enters the vehicle is actually not a weapon, it is a big tool that can be used ONLY on vehicles that are damaged, the "tool" fixes the armour around it.
NOTE: There are turrets called multigunner turrets that are stationary, and have the same effect.