Your Big Guns has been permanently increased your critical damage damage with Heavy Weapons by 25%.
Make your way inside the Vault. This location has been taken over by the Gunners. There are a number of traps throughout the area and more in the entry way.
Rush into the Vault and go into the elevator in the area.
Once inside, try to get into the first room on the right (It is protected by an Advanced Lock). Inside is a terminal that will let you disable the nearby turrets.
There are also 2 laser trip mines. Either disarm or shoot them to disable them.
Head past the suspended cans and into the first room. Immediately turn to the right and go through the door there.
Rush past the suspended grenades and turn to the left. Take the stairs there down into the Residential area.
Head to the right through the second door at the bottom of the stairs.
In this room go to the right. Clear out the Gunners in the area.
Go to the North.
In the next room, directly in front of you on the table is the Big Guns Bobblehead.
"The best way to win an argument is to be the loudest."