Which details about the gun were unusual and noteworthy for Sergeant Wilson and Mason?

From the text:

Lying across his chest was a curious weapon, a shotgun with the barrel sawed off a foot in front of the triggers. It was clear that this had been fired at close range and that he had received the whole charge in the face, blowing his head almost to pieces. The triggers had been wired together, so as to make the simultaneous discharge more destructive.