Another option for a little money is if your shot gun has changeable chokes is buy a rifled choke tube. If you go with the rifled choke tube be advised the choke will tighten tighter when the slug travels thru it.
Use a good anti seize grease or slick 50 grease and loosen the choke every couple shots.

Now if you were going to spend a bunch of money like some suggest I would buy a modern inline muzzle loader. Can be used during you shot gun season and then used during the muzzle loader season.

Those new Inlines are pretty darn good.

Al