Shorter barrels are usually stiffer and shoot better and longer barrels shoot faster but there is no hard and fast rule with lenght v velocity v accuracy. Every barrel has it's own personality.
Look at the length of benchrest rifle barrels and they're all short.
With that said, the 338 L isn't a benchrest cartridge but rather a long range bomber so velocity is equally important as pin-point accuracy.
For the above reasons, based off velocity demands, longer than 24 is the norm and 24" would be considered quite short in the circles who shoot the big rifle.
If I built one, it would have 26 or more and be relatively stiff similar to a #3 heavy sporter Bartlein....or stiffer.
Just my opinion,
Zeke