The SPLINTER RPG actually has two firearms, one called a Blunderbuss, and one called a Blunderbus (sp) in the upcoming Sometimes Little Wondrous Things sourcebook. This isn’t an error or an accidental duplication. These are two conceptually and mechanically distinct guns. The Blunderbus with one s is a sawn-off single shot 10 Gauge shotgun affectionately referred to as a “Blunderbus”. The correctly spelled weapon is the real thing, circa the 1800s or whatever.
Just, y’know, in case you were wondering.