TOM, it depends on the specific cartridge you are talking about. If it is actually a 300 Remington Ultra Mag (RUM), then it is a Magnum Length Action, if it is a 300 Remington Short-Action Ultra Mag (SAUM), then it is a Short Action. FYI, If it were a 300 Winchester Magnum, then it would be a Long Action.
Cheers.
There's no industry standard on just what constitutes "short" and "long" actions. Most bolt-action rifle manufactures make at least two lengths, some three; Sako bolt actions come in four lengths to suit different cartridges.