The .338 Remington Ultra Magnum was introduced in 1999 and is based on the .300 Remington Ultra Magnum case necked to accept .338 inch bullets, but with the shoulder moved back and with a slightly shorter overall case length. The large capacity case is beltless and is a modified version of the old .404 Jeffery case.

Advertised ballistics propel a 250-grain bullet at 2,860 fps from a 24-inch barrel; however, in a Remington Model 700 fitted with a 26-inch barrel velocities exceed 2,900 fps with several powders, while staying well below SAAMI pressure guidelines of 65,000 psi.

Case life has proven to be long, as long as loads are kept within pressure guidelines. One problem during load development did occur on multiple occasions with cases that had been fired and reloaded multiple times. As the primer pockets loosened, primers from loaded cartridges that were in the magazine and subjected to maximum load recoil with 225- and 250-grain bullets would dislodge and fall from the case into the magazine. It is therefore suggested to use new or once-fired cases when handloading cartridges that must be ultra reliable in the field.

Due to the .338 RUM’s large powder capacity, a Large Rifle Magnum primer is strongly suggested, with Remington 9 ½ Magnums being used to develop the accompanying data.

Top performing powders (in accuracy and in producing top velocities) included Hodgdon H-4350, H-1000, H-4831; Accurate AAC-4350; Vihtavuori VV-N165; Alliant RL-19, RL-22 and RL-25.