How about either 50 Alaskan or 500 Linebaugh, loaded WAY down of course!!!! Starline makes occasional runs of both.

Quickload shows the big Alaska case getting 850fps with a 450 grain bullet in 7" barrels at 18k psi with Trailboss. My original Wilkinson only gets 710fps with 70 grains of 2F and a 450gn bullet. The software shows the snider case has about 85 grains of water "volume" under the bullet and needs a filler wad whereas the Alaskan shows about 75, so I'd imagine it would give about the same velocity with BP.

With the smaller 500L case a bunch of fast powders like unique show 800-900fps at <=18k psi, but black powder would just seem more fitting.

Bob