Bobblehead - Big Guns is one of a total of twenty Vault Boy Bobbleheads found throughout Fallout 3. It permanently increases your character's Big Guns skill by 10, to a maximum of 100. The inscription on its base reads "The best way to win an argument is to be the loudest".
In the basement of Fort Constantine's CO Quarters. To reach it kill the protectron which is usually found there and then proceed down into the basement. As you go down you will see a open safe on your left (literally open, not just one which has been unlocked). The bobblehead can be easily found standing in the safe along with a few other items and some caps. It's inscription reads "The best way to win an argument is to be the loudest". The bobblehead will be presented as the vaultboy holding a minigun.
Obtaining Bobblehead Big Guns