I thought about building a 338win mag with a 20" barrel for elk and moose hunting. I find very long barrels (~26") to be in the way with really thick hiking like I do in Newfoundland. A short handy barrel would work out good I believe.
For a "short" 338Win.Mag, how about 23"? I don't know, but I wish they would settle on
different lengths for std. and magnum rifles. Personally, I like 23" for magnums, 21" for the standards (270, 280, 308, 30-06), and 19-1/2" for carbines (not 18-1/2").
Yeah, I know
Anyway, concerning the orig. post, shortening from 24" to 20" in the std. cartridges noted shouldn't be too bad. I'd expect a loss of about 100 to 150fps, depending on the
cartridge.
The 308Win. seems to hold it's own pretty well down to 20". I found the following noted by Finn Aagaard in his book, "Hunting Rifles & Cartridges". He tested various loads using a 308Win.: "The results of the chronographing were quite interesting. The five different factory loads recorded velocity losses varying from 20fps to 35fps when the barrel was shortened to 20" ..... In contrast, shortening the barrel a further 1-1/2" to 18-1/2" produced velocity losses that averaged 71fps. The average total velocity loss for 12 different loads when the barrel was cut from 22" to 18-1/2" came to 96fps. It seems therefore that one would lose almost nothing by shortening a 308 barrel to 20", but that cutting it shorter does exact a price."