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Click on figure to access ANSI technical cartridge drawing

A commercial cartridge, the largest industry standard 30 caliber cartridge. Introduced by Remington at the 1999 SHOT show, as the first of its Ultra Magnum cartridge series (338 RUM later introduced in 2000, the 7mm and 375 in 2001).

By all definitions, as a hunting cartridge or sporting cartridge, the 300 RUM is large. Dimensionally, the RUM can be thought of as a full 375 H&H length but beltless case, with a 375 H&H rim, and a 404 Jeffery plus diameter body, sporting a modern 30 degree shoulder and a full caliber length neck. Capacity wise, the 300 offers 110.2grs (of water, full case), or 100.9gr with a flatbase bullet seated .500". In comparison (likewise, full case and .500"): the 300 Weatherby offers, 91.0 and 81.6; the 300 Win Mag, 87.0 and 77.6; the 30-06 a "mere" 67 and 59. The only semi-standard 30 caliber cartridges offering greater case capacity are the 30-378 at 121.3 and 111.0, and the 30-338 Lapua, at 114.8 and 105.4grs.

Given the levels of energy the RUM can impart to bullets, a heavy firearm is highly desirable, and even then recoil can be quite stiff. Given the RUM's case capacity, 28 or even 30" barrels are necessary to achieve reasonable efficiency. The RUM can drive a 125 gr bullet at 22-250 velocities - 3850fps. Likewise it can drive a 165gr at 3400 fps, generating 4200 fp muzzle energy; a 180gr at 3325 fps, or even a 220gr at 3000fps. These levels of performance only slightly trail either the 30-378 or 30-338 Lapua. For the 1000 yard competitor, these velocities ensure that all competition bullets will stay well supersonic to the target and well beyond. Even with hunting bullets this can also be the case: A 3450fps 150gr Scirocco will be still at 1505 fps at 1000y, and won't become subsonic until beyond 1300 yards. A 180 Scirocco launched at 3250fps will still be running at 1575 fps at 1000 yards.

The nominal bullet diameter is .308". ANSI lists the maximum operating pressure as 65 kPSI (1kPSI greater than the 300 Win Mag).