About 10 years ago, I was considering buying an 1874 Sharps reproduction, as made by one of the two U.S. firms in Montana (can't remember which one).
Anyway, I was surprised to see in their catalogue that they did NOT recommend ANY smokeless powder with the .45-120 Sharps case.
As I recall, the manufacturer said that the use of smokeless powder in such a voluminous case caused dangerous pressure spikes, and would void the rifle's warranty.
Perhaps that was with the powders of the day (early 1990s).
Accurate Arms' XMP5744 bulks up well, and perhaps it can be used with the .45-120 today.
But if I owned a .45-120 I'd contact the manufacturer for their take on using smokeless powder.
Not to be an alarmist, but it might be worth checking out.