Set up a class with One Man Army as your first perk. Head into a game and go to a mounted gun. Use One Man Army to select a shield class, either the default or one you make yourself. Now while your class is changing, use the mounted machine gun. Once you're done switching classes the shield will be out guarding you, but you WILL NOT be able to see it. Other will be able to see it though, and it does deflect bullets.

Very useful for getting the 5 kills while on a mounted gun, or if you just like to use them in general.

http://www.mapmonkeys.com (less info)