Sound more like a hand-canon.
Single shot, short barrel, break action that fires a variety of different projectiles. That's pretty much the description of the M79.
There are anti-personnel 40mm grenades that turn the M79 or M203 into a "shotgun". They aren't really used anymore because we don't use the M79, and the M203 is mounted below the barrel of an assault rifle. No need to carry antipersonnel 40mm grenades when you already have an antipersonnel rifle.
What about a shotgun that shoots shotguns
Why sir, you think too small. Think fractals.
think of it more as a hand held 75 mm gun