.300 Remington Short Action Ultra Magnum Ammo (also known as 300 RSAUM, 300 RSUM or 300 Rem SAUM) is a .30 caliber short magnum cartridge that is a shortened version of the Remington 300 Ultra Mag, both of which derive from the .404 Jeffery case. The Remington Short Ultra Mag was put on the market shortly after Winchester released its 300 WSM round in 2001, resulting in the Winchester product getting the marketing advantage that has eclipsed the Remington offering
300 RSAUM ballistics are similar to 300 Win Mag and 300 WSM. The difference in ballistics between the WSM and RSAUM are insignificant in all practical applications. Differences in muzzle velocity and muzzle energy are related to barrel length rather than case dimension.
The WSM has slightly more case capacity and thus, slightly more velocity
The WSM is more popular and more ammo and handloading data are available