I would estimate the .460 case, at 1.8" holds 62gr. 3F GOEX BP. poured into an unsized case from my horn. If tapped to settle or a long drop tube used, then 1/10" compression it would hold about a 250gr. bullet., with a shallowly seated cast bullet. The free bore will allow that & likely allow a longer, heavier bullet. It will not, however be an express (high vel) round, but will have enough power for deer.
My Sharps has a 1.9" chamber with 3/8" freebore. I load a 525gr. RN cast bullet with 61.0gr. BP. I haven't chronographed this load, but I have tang sight, sight settings to 600 meters. I won the long range event at Rendezvous with this rifle & load, mostly offhand shooting, a few standing or sitting cross stick shots.
There were 25 shots & I hit 22 of them. Range finders allowed, but ranges were marked - some incorrectly. Longest shot was 650meters, shortest shot was 18 meters at a golf ball laying on the ground. All the rest of the targets were steel.
Oops, bit of crowing going on there.
So - 61gr. or 62gr. of powder will work. I suspect 1,150fps with the 525gr. bullet - 30" bl.
That load and a 300gr. bullet should be making about 1,300fps.