I would say send it to Bobby Hoyt, and have him bore it out as far as it's safe to do so. I doubt seriously that .50 caliber is achievable. But, somewhere between 0.470"-0.485" probably is.
Thompson/Center used 0.005" deep grooves, so the groove diameter is already 0.460". If SDsmlf's measurements on his Cherokee barrel apply across the board to all Cherokees.....
13/16" diameter octagon barrel = 0.8125"
0.8125" - 0.460" = 0.3525"
0.3525" - 0.125" = 0.2275"
0.2275" ÷ 2 = 0.11375"
0.11375" = barrel wall thickness/.45 caliber rifle
0.8125" - 0.470" = 0.3425"
0.3425" - 0.125" = 0.2175"
0.2175" ÷ 2 = 0.10875"
0.10875" = barrel wall thickness/.470 caliber SB
0.8125" - 0.485" = 0.3275"
0.3275" - 0.125" = 0.2025"
0.2025" ÷ 2 = 0.10125"
0.10125" = barrel wall thickness/.485 caliber SB
0.8125" - 0.500" = 0.3125"
0.3125" - 0.125" = 0.1875"
0.1875" ÷ 2 = 0.9375"
0.09375" = barrel wall thickness/.50 caliber SB
The above calculations are dependent upon neither the front sight dovetail, nor the barrel key underlug dovetail, nor any other screw holes; being deeper than 0.125" deep.
Track of the Wolf usually has the .50 caliber muzzleloading wads, measuring 0.510" in diameter, in stock. Others here can speak to the viability of using a 0.510" diameter × 0.025" thick over shot card wad in a bore that's 0.0.25"-0.040" smaller in diameter. Of course, in a .50 caliber SB barrel the wads are the correct fit.