Not bad for a guesstimate.
"So what is the ideal barrel length for a 338 Lapua Magnum?
I would say anything about 26″. If the length isn’t an issue, you may be able to work slower powders into higher velocities at the longer lengths. While the 17″ barrel still performed better than some other options, it didn’t allow the cartridge to reach its full potential. I don’t know of anyone with a 20″ 338 Lapua Magnum that is happy that they went short."
https://rifleshooter.com/2017/03/338-lapua-magnum-barrel-length-versus-muzzle-velocity-30-17-inches/
