The "Brian Gun" is a heat-seeking missile in the car racing/shooting game "San Francisco Rush: 2049". The multiplayer part of the game is similar to Mario Kart's battle mode. One of the items is a pack of 3 Homing Missiles. They kill you in one hit. Brian likes them. Brian utilizes them to great effect. The item is now called a "Brian Gun" because of this. It's a cheap jerk item that breaks the game, and he tries to find it every time it spawns. Any comparable thing in a game or real life can be referred to as a "Brian Gun", especially the portal gun in Portal (Play Portal Dammit!). He uses it ALL THE FREAKING TIME! Use another gun!