I don't think a cap/primer fired mortar is "illegal" as long as the tube/total apparatus meets the definition of a non restricted firearm as in over 26' in length. A percussion fired tube is a gun, even a deck top cannon with a percussion firing mechanism would be a restricted item. A fuse fired apparatus falls under some sort of "antique" status similar to matchlocks with no size requirements.
Don't ask me for the "appropriate" act sections as I don't have them, I'm just repeating what I was told by a CFO at a cannon demonstration one time.
I think the "exploding air burst" thingies may cause some excitement among the more knowledgeable powers-that be. I one time mentioned that I would like to load my cannon up with a hand full of pistol bullets and give a high flying flock of geese a try, I didn't know there was a fish cop in the group and he got my undivided attention for more than a few minutes explaining why that wasn't a good idea...