Laws and RPG used to be like cops. You could never find one when you needed one. So we had a contest so everyone could have an anti tank weapon.
Here ya go: The winner of the Ultimate Anti Tank Weapon Contest.
Had to be under $20 to build so every person on the field could have one.
I have played in my tank against a SQUAD of guys with Nerf launchers that would not shoot any farther than this. They killed me all day long. After a while I stopped TRYING to penetrate deep into enemy territory even WITH infantry support. We killed a bunch but like mosquitos, there was always ONE MORE ready to nail you.
And as long as the field or producer says it is legal. Its legal. I have YET to see a rule book that says a cannon has to be pneumatically powered.
The runner up actually shot Nerfs:
Go buy 2 foot of 2 inch diameter of schedule 20 PVC pipe. The thin wall. Kind of hard to find in the city where they have building codes. Schedule 40 works too but about twice as heavy.. You could drill holes in it.
Get a 2" x 1 1/2" reducer fitting and glue it on. Glue 2 feet of 1 1/2" PVC pipe to it. A cap on the end so it doesn't slip out of your hands.
You should have something that looks like a 4 foot whiffle bat. Drop in a Nerf and swing it like a baseball bat. See where that Nerf winds up. Then repeat the process until you get it to go the direction you want...
Trim the nerf fins to match the bulb diameter so they drop in and slide out easily. Tinker with the length of the 2 inch pipe, maybe a cut out the 2" section so it throws like a Lacrosse stick. Might be faster to reload too.
Do a double ended version. Whomp! Whoomp!
Sounds like an afternoon of silliness and fun to me. Just add beer, a pizza, and some buddies.
Just Remember, REAL Scenario player don't run AWAY from tanks...