You could always call it a 2 pounder. After all who is going to tell you you are wrong? 
Why did I know it was going to be a Maverick? Lovely work, by the way. Have you considered modifying a Rapid-Fire-20? It would need considerable cosmetic work, but the noise it makes during operation is enough to drive people to take actual cover.
Also, the genre with supernatural horror is sometimes called ‘Screampunk’.
I measured mine, and estimate it as a .65 caliber shell, with a .80 barrel. However, the shell is long enough to be a good solid two pounder, I would agree. Maybe pound and a half.
Did you remove the little projection so the barrel swings out fully?
heh. The maverick was the first. I’ve done another brand that has a tommygun style that does the same thing. Looks awesome. I did indeed mod the chamber so it swings and spins.
How about a Royale with Cheese[sup]TM[/sup]? 
There is a lovely off-brand two-barreled pom-pom gun I’ve seen at Toys R Us that might be interesting for you to mess with. It feeds from two clips.