Bobblehead - Big Guns
"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 an open safe on your left (literally open, not just one which has been unlocked). The bobblehead can be found standing in the safe along with a few other items and some caps.