I'd say 21 inches is too short for the 338 mag. The 9.3x62 or .35 Whelen, maybe, but the magnum's big case and Sako action makes the short barrel sound like a crazy proposition. Almost all centrefires will give better velocity with a longer-than-usual barrel but we have come to make compromises based on relativities. Where there is a smaller case-to-bore capacity ratio, such as in the 375 Win. or 9x56, the velocity loss per inch trimmed and the noise and rocket-blast are understood to be reduced, making short barrels more acceptable.

The only point of such a short 338 I can think of would be if a short rifle is needed for transport and the availability of factory ammo is crucial.