All of my load books for non magnum cartridges are with 24" barrels. Some magnum data is from 24" barrels, some 26". You just have to look. I don't currently own anything with a 24" barrel, but those that I did were within 10 fps of what the book said they should be.
Most of my rifles are 22", but some as short as 18". I work up the load with the 22" barrels and usually find my speeds with a max load are within 50 fps of what the book says they should be with a 24" barrel. That is about what I expect with a 308 class cartridge from a 2" shorter barrel. I then shoot them in the shorter barrels just to see what I'm getting. My 18" 308 BTW is about 50 fps slower than the 22" rifle.
There just isn't nearly as much loss from shorter barrels as a lot of people think. Based on what I've seen these are the numbers I expect with barrels between 20" up to 26". As you go longer the differences are smaller. Get below 20" and you see greater differences.
30-30 class cartridges: 5-10 fps slower/inch
308 class cartridges: 10-20 fps slower/ inch
30-06 class cartridges: 15-25 fps slower/inch
30 caliber and under magnums: 25-35 fps slower/inch
magnums over 30 caliber will be in about the same class as 30-06
