The 7mm RUM was created using the .404 Jeffery casewhich was also used to develop the .375 RUM .300 RUM, and .338 RUM. By necking down the .300 RUM to suit the .284 or 7mm projectile, Remington produced a non-belted case with a head diameter that is somewhat larger than belt diameter of original belted numbers. The resulting case has significantly more capacity than any conventional belted magnum. Compared to the 7mm Remington Magnum, top 7mm RUM loads deliver 25% more energy at 300 yards. Such performance demands a price and in this case, that is a large muzzle blast, sharp recoil and short barrel life.
The 7mm Ultra Magnum boasts the largest case of any commercial 7mm cartridge.