one of the last times i went shooting at a public range (merely state land that allows shooting), there was a guy there that had a small mortar cannon.
he built it himself, nicely machined, with a nice wooden base, elevation adjustment, etc. this thing fired concrete filled tennis balls.
one of the coolest things i've ever seen.
i've considered building one myself.
is this something you have plans for, or are you just gonna wing it??