JimG,
You can shoot the fosters out of a rifled barrel, you will need a good lead removing solvent and a TON of elbow grease to get it out. A smooth bore will lead up after awhile too, just easier to get the lead out. The flush mount rifled choke tube does impart spin on the slugs as evidenced by a Rottwiel slug that was recovered from just under the hide after going through the spine and off shoulder on a steep quartering buck that my son shot. It indeed had the rifling ingraved into the lead collar at the base of the slug. The better rifled tubes are the extended ones that are about 5" long. Browning used to use these in their older slug guns, and ones like them can be purchased in Rem choke from some after market choke tube manufacturers.
I think you would be surprised with a smoth bore and the right slug for the job. You didn't say how far you plan on shooting? Most smooth bores with good sights are capable of shooting out to 75 yards with pretty good accuracy, 100 yards is pushing it unless the slug is stable. Some slugs are only meant to shoot 50 yards and have a mind of their own after that. The cheap Wolf slugs shoot good out to 50 yards just like they were designed to do, but after that who knows where they are going, even through a rifled bore.
Get some Rottweil Brennekes if you can find them and some Winchester, Remington and Federal Fosters. There are also lots of others like PMC, Wolf and Sellier & Bellot to name a few.