A smoothbore that would use the Lyman .662 ball with a heavy patch would be a very good choice. Though I suspect the 20 will do well for deer sized animals as will a 24 bore with a .535 ball.
The 16 is the best compromise of recoil and power. The problem with smoothbores is that they sometimes like heavy charges of powder. My 50 smooth barrel shoots 2/3s the powder of the 16 bore to shoot best at 50-60 yards. A 16 that likes 140-150 grains in a light fowler will not be pleasant to practice with.
In guns under 9-10 pounds it can make a lot of recoil in the 1600 fps velocity range.
My 16 bore rifle, stocked as a circa 1800 English HS (think fowler buttstock) took some getting used to. But I never notice it now especially on game.
Nor should it be shouldered as pictured below. One must shoot it like a shotgun and stand more facing the target. This is not the best for offhand accuracy but its best for heavy recoil. If this rifle slips off and hit the biceps the shooter will know it for quite some time.
This said this the the best ML hunting rifle I have ever owned and I have and have had some pretty good rifles. It is far handier than a long barreled fowler even at 10 pounds.
This rifle has a 80" twist, I suppose I should pattern it with shot at 20-25 yards just for fun. I have some wads.
Dan