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. Further down there will be an open safe on the left (literally open, not just one which has been unlocked). The bobblehead can be found standing in the safe along with a 10mm pistol and ammunition along with the Fort Constantine launch codes and some caps.