Mainiac
I have had a TC New Englander in 50cal with an original 12ga barrel for some time that my 17yr old son uses. It is of my opinion that one does not need a high volume of powder to that a deer at least in my experience. I shoot 3f in all my 50cal rifles one of which is a James Klein built flintlock. As I mentioned before one does not need such a heavy load. I have found around 50 grain to be more than enough. My son took a 170# doe @ 50yds with the New Englander using only 45 grains of 3f.
There is a difference as mentioned between pressure build up of 2f and 3f. From my research of the matter it is closer to a 15% difference in volume.
Now at last I looked you can still purchase a 12ga barrel from the TC aftermarket custom supply. The difference in buying an original and the after market is that the originals had removable choke tubes. That way one could hunt a variety of game and also install a cylinder bore choke and shoot round ball as well making the weapon even more useful.
Again I do not believe such heavy loads are needed. As for 3f, like I said, is all that I use in my flinters, even my 42" barrelled 20ga/62cal smoothie, I use a charge of only 50-55grains for round ball and/or shot on pheasants, turkey, and deer.
PS you will most likely pay as much or more for the 12ga barrel as you paid for the rifle complete. I did, for I found mine for the same price as you and paid $120. on evil-bay prior to their anti-gun movement for the 12ga barrel.
FGV