The 300 Remington Short Action Ultra Mag (RSAUM) was introduced along with the 7mm RSAUM in 2002. Following hard on the heels of the Winchester Short Magnums (WSM) these cartridges are similar in performance to Winchester's cartridges in like caliber and seem redundant, but there is a purpose: The WSM cartridges are slightly too long to be housed in Remington's Model Seven, so the 300 RSAUM has a slightly shorter (2.015-inch) case. This also allows it to be adapted to AR10 semiautomatic actions. The 300 RSAUM, like all of Remington's Ultra Mag family, is based on the 404 Jeffery case, greatly shortened for the RSAUMs. The result is a short, fat case that burns its powder efficiently and is conducive to accuracy. Currently only loaded by Remington, the 300 RSAUM features a 150-grain bullet at 3200 feet per second and a 180-grain bullet at 2960. It is suitable for any hunting for which a fast 30-caliber is applicable...which is darn near anything short of really large game. —
Craig Boddington