I have had several Huntsmans. Yes, you can get a shotgun barrel, but it is a muzzle loading barrel, not for cartridges. They haven't made these for a while.
.You will be told the story of the breech plug blowing out, don't beleave it. Just open carefully in the event of a hang-fire.
IIRC the .45 is a 1:48 twist, a little slow for the sabot, but fine for round ball, or Lee R.E.A.L. bullets.
It should do fine with pellets.
The extendable ram-rod is a little weak, be careful with it.
.The ones I had were very accurate. At the time, Virginia did not allow in-line for hunting, so I sold mine.
Tractors are made to work!
"A Cub will do as much as a team of horses,.. More in hot weather!" - C. W. Spradlin 1909-1994