Sounds like you've got a winner - good shooting - great idea for a single shot. Too bad the doubles lack the barrel size to do this - BUT - full chokes could be rifled?

Here's a group shot with my 16 bore double, using the right, straight rifled barrel only, using - .684" bore round ball, 1-.715" card, 2, 1/2" BP lubed 16 bore felts, then the ball and crimped with a paper case cone crimp starter from an old press. Oh yes, I used 3 drams (82gr.) 3F GOEX - & should be running close to 1,300fps. The straight rifling will restrict longer range accuracy, but the 1st 5 did go into 1-1/2", offhand at 28 yards. I was cold, tired and shot quickly & I'm not as a good an offhand shot as many guys - enough excuses? Not sure that #6 hole is mine - that's when I noticed it, though - so could be. The carboard sheet was laying in the snow as I'd left my targets in the truck - too much crap to pack for the walk-in just to see if the gun would go bang. I should note here that the rifled barrel is oversize from chamber to muzzle, has a .671" bore diameter, with .703" groove diameter, = .016" deep rifling.
Large bores & round balls are fun! Keep up the good work, Dave.