I'm going to try for some better pics later today. The barrel is a modified muzzle loader barrel. I'll get back with all the dimensions when I get the photos taken. Previous owner says 56-52 on the caliber, but I cast the camber and find out for sure. It feels great in the hand and probably sat well on the cross sticks.
As to why the barrel is so heavy, the first answer is probably thats the barrel the smith had around. 2nd was hopefully it would give the gun a boost in accuracy and also the ability to shoot more before it had to be set down to cool off. Finally a lever action Spencer like this with a barrel this heavy on a set of cross sticks could be worked quite quickly putting a lot of lead down range in a hurry. If the hunter had a few blakeslee tubes, or perhaps a few whole boxes picked up from his ex-cavalry buddies I could see this been a very effective Buffalo hunter. Empty the mag down range, roll it over on the sticks, reload, empty, repeat.