I personally would look at a .54 caliber that shoots roundball. They recoil is not all that brutal because of the large caliber. If you went with something like a Lyman Deerstalker or Trade Rifle, they are not all that long. The beauty of a muzzleloader is you can load them down, still keep them accurate, and lower the recoil at the same time. Granted you might have to adjust your range you are willing to shoot, but even a .54 caliber roundball because of its mass, with a even 60 grains of powder should be lethal out to 75 yards, because of the large ball, while at the same time not beat you up.

Try and get to a range and see if someone has a muzzleloader. Ask them if you can shoot it with 60 grains and a roundball or ballet. I think the recoil will not bother you. My eight year old nephew shoots a rifle like that all the time.