From Homestar Runner Wiki
Bubs's shotgun was used by Bubs to destroy the Compy 386 to save the Homestar Runner universe from The Virus in virus. Strong Bad was less than grateful. Due to the nature of The Virus, this gun first appeared as one of Homestar's legs. It resembles a Winchester Model 1912 with a sawn-off barrel.
An interesting side-note, the hole in the Compy 386 is larger than the hole in the wall. A normal shotgun would spread the pellets out as they traveled through the air. (Although it could be implied that the pellets' energy was reduced from the impact with the computer so only the centermost (fastest moving) pellets actually blew through the wall.)