Callahan's Magnum in Broken Steel
When you are asked to blow up the Enclave base on your last mission, go to the satellite. Rather than blowing it up, blow up the Citadel. Sentinel Lyons will still pick you up. After you complete that, they will try to make contact with the Citadel but it will not go through. They will go directly to the Citadel, see all the remains, and confront you. After that, enter the Citadel courtyard to find a huge hole in the middle. Drop down all the way and look for a door that says "Armory". Go inside and quickly look behind the little "office". You should see a safe and a terminal. Open the safe to obtain a powerful .44 called Callahan's Magnum. Its damage is 66. After that, search around and you should find the Armory very easily, with many weapons and ammo.