RECOILLESS CANNON CONCEPT

I was thinking one day, y'know a recoilless rifle would be a quick and easy way of coming up with artillery that doesn't involve muzzleloading blackpowder cannons.

Then I thought and "oh, rifling."

Rifling means machine shop work, and I'm aiming to use relatively common drawn over mandrel steel pipe of large caliber... so I thought a bit and then-why not use a finned sabot?  It's good enough for the M-1 Abrams 120mm smoothbore.  So I came up with this-then I remembered the Foster shotgun slug is meant to be shot out of smoothbore shotguns-that would increase potential payload... at the expense of velocity, range and a more pronounced trajectory so I amended.

Other than that it's a recoilless cannon-not a rifle, this is a smoothbore.  Slap some large DOM seamless pipe together, and yes you will need to machine a breechblock.  This is a conceptual drawing and not meant to be an actual blueprint.  You finish that but it's got to beat using that Civil War re-enactor's Parrot Rifle.