The 300 Remington Ultra Mag (RUM) was introduced in 1999. It is based on the 404 Jeffery case necked down, and is a full-length magnum (2.85-inch case), meaning that it requires a 375 H&H-length action to house it. The fat, unbelted case has a rebated rim of .534-inch, base diameter of .550-inch, and headspaces on the shoulder, considered more precise and thus more conducive to accuracy than belt headspacing (as belted magnums do). The 300 RUM has greater case capacity than the 300 Weatherby Magnum, thus is potentially faster. Remington's factory loads feature 150-grain bullets at 3450 feet per second, 180-grain bullets at 3250, and a 200-grain Swift A-Frame at 3025. Remington remains the primary source for factory ammunition and production rifles, but due to its accuracy and velocity the 300 RUM is becoming popular among long-range shooters and serious mountain hunters, and is chambered to an increasing number of custom and semi-custom rifles.
