It resembles a firearm made from a matte, lustrous greyish metal (or metallic material). The lines are roughly like a Mossberg 500 in pistol grip configuration, but squarer in cross section. On picking it up the first thing most people notice is its weight. Further inspection will disclose that the chamber is apparently unopenable, and the trigger apparently loose and non-functioning. The casing and tracery are very carefully crafted, although the patterns are oddly difficult to remember accurately and even appear to change.
It is only in the hands of the Mutt that this artifact becomes a deadly weapon. It does not do much in the way of destroying buildings, but it can be aimed very specifically -- potentially down to a genetic level, not to mention at bits of mind and soul -- and fired very subtly. It can mimic the effect of a normal firearm too, with very many different types of 'ammunition' and apparently without the need for reloading. All in all it is the weapon of a godmoder, which is why it is not appropriate for use on a serious storyboard.
The pump action, however, really is only for show.
The Shotgun is called, in an ancient, made-up language, a name meaning That which accurately targets [falsehood/arrogance]. Another language calls it Dog-Barking-Judiciously, and a third names it Ennui (spelled PWNLI).
The Silver Shotgun has been known to manifest as a Silver Gatling Gun and various other Silver weapons. It has never seriously attempted to be a sword, knowing its author's views on the subject.