Caliber is not the problem, rifling is.
A rifled gun makes a terrible shotgun'
I have spent years experimenting with muzzleloaders and modern guns with rifled barres and shot. .45 , .50, 58, 38, .22 .44. .45. calibers.
Nothing but a waste of time if your gun has rifling. Remove the rifling and you have a serious rabbit killer.
Bobby Hoyt will remove the rifling, but not on a CVA due to their breech plug design.
There are a couple of videos on youtube shooting shot from a muzzleloader. One by Dave Canterbury. You can do it but it is more a survival scenario. If you have the rifling removed, let us know who did it or how it was done.
That would be the most sensible thing.
Even if you had a 50 cal with a smooth bore you probably wouldn't be able to swing it fast enough to hit a running rabbit anyway if you are bunny busting. Sitting rabbits you can rifle with a ball.