I think a rifled 12 bore would be fun - or the .50 2 1/2" you mentioned, rather than a .45 cal.

The .50 would give you all you needed, I'm sure - 1,900fps to over 2,000fps with over 500gr bullets sounds about right, considering the .50 Alaskan runs about 1,900fps with a 2.1" case.

Of course, the .50 Alaskan would also be a good choice with easily converted brass. Starline USA)sells ready made .50 Alaskan brass.

I have used my 6.5x68 dies for sizing and crimping bullets for the .50 Alaskan and .50/70 Sharps rounds.

In thinking this over some- I'll edit this response to include the .50/70 Government round.
if chambered up for shortened .50 Alaskan cases, .348 brass can be used.

It would make a nice 1,600fps round for 520gr. lead bullets.