The only "why bother?" question I can think of is, why did someone build up a flare gun so much. Shoulder stock? So you can aim it really well when you're firing a flare into the sky?
The existence of this weapon is making me suspect that 26.5mm (like 37mm) is more than just a flare spec. I wouldn't be surprised if it has riot-control applications too ... batons, rubber shot, CS, maybe something that goes *scream* instead of *bang*. 12-gauge and 37mm are that way, so why shouldn't 4-gauge (aka 26.5mm) be?
In any case, it would be a lot of fun to get the tax stamps and do the project the OP suggests, but I don't have that kind of time or money.