It's an interesting question and would be nice if there was an interesting answer! One of the factors that drove the .458 Win into disrepute was the shorting of the barrel from the original 24" to the current 22" version. Admittedly, we all know that Olin/Winchester down-loaded the cartridge as well because of the problems with caked ball powder in the heat of Africa but if the "rule of thumb" that you loose 26 fps for each inch of barrel holds true with the Win as much as it does with other cartridges (and I don't know that it does, having never owned one) then the loss is 52 fps. That isn't much most of the time but when your bullet weighs 500 grains, it makes a significant difference. Like .475 says, the only way to tell is to start with a long tube and cut it down, chrono-ing all the way. That's an expensive way to find out and when you get down to 22" and find you aren't happy, you can't put it back!