I've worked with 2 .45's with 3 1/4" cases along with a .50, 3 1/4". As to finding loads that are accurate using black powder, it was trouble in the long cases. When using smokeless powder, fillers ( I don't like fillers very much) were necessary due to the huge black powder-type cases not being designed for efficient smokeless powders.

Dan Pharris can attest to the problems with th e.45 3 1/4" in load developement and ringed chambers that happened.

I am well aware of the .45 3 1/4 Nitro Express round, which is loaded by a number of guys here, well one or two, and seemingly successfully- I'd jsut rather work with a shorter case - easier to find a good shooting load, and capable of almost matching the longer cases with today's powders- loving no more than 100fps, even in a case that's only 2.4" long, like the Sharps version of the Winchester .45/90.

It was called variously by the load Sharps used in it, .45-110-500, or the .45-100-550.

Today's brass holds roughly 95gr. to the top of the neck and with a long throat, 3/8" or so, seating can be long and not interfere with smokeless loads. The useable capacity if the bullet is seated out, will be greater than the .458 Win Mag. and results could be close to the longer case, but at reduced pressure.

The 2 7/8" was suggested as it will give similar ballistics as the .45 3 1/4 Nitro Express with about the same pressures, using today's powders.

I'm just guessing here, but Accurate Arms data should help in this regard.