6mm-204 RR (rifle loads) (Handloader issue #344)

6mm-204 RR (Rifle Loads)

The 6mm-204 RR is an efficient American wildcat rifle cartridge developed in 2011 by Hodgdon ballistician Ron Reiber. Its design is based on the .204 Ruger case, which itself was derived from the longer .222 Remington Magnum case. To create the 6mm-204 RR, the case neck is expanded from .20 caliber to accept standard .243-inch bullets while its basic body and shoulder dimensions remain unchanged. The “RR” suffix recognizes Reiber’s role in developing the cartridge. Although similar experimental cartridges—including the 6mm-222 Remington Magnum and 6x47mm—had appeared decades earlier, the 6mm-204 RR combined that established concept with modern .204 Ruger brass, newer propellants and a much broader selection of specialized 6mm bullets.

Reiber and custom stockmaker Rod Herrett developed the cartridge primarily for hunting rockchucks and other varmints with compact, highly accurate firearms. The .204 Ruger offered excellent accuracy and high velocity with light bullets, but its small projectiles did not always produce the desired terminal effect on larger varmints, particularly when fired from shorter barrels. Expanding the case neck to 6mm allowed the use of substantially heavier bullets without requiring a much larger powder charge. The resulting cartridge delivered greater retained energy and more decisive performance while preserving the mild report, low recoil and efficiency that made the original .204 Ruger case attractive.

Forming 6mm-204 RR cases is comparatively straightforward for a wildcat. Readily available .204 Ruger cases can be expanded to 6mm in a single necking-up operation with the proper forming die. This eliminates the extensive case reforming and fireforming procedures required by many other wildcats. Case-neck thickness must still be checked carefully because it can vary among brass manufacturers, and tight-necked custom chambers may require neck turning to provide safe bullet clearance. Since the cartridge has never been standardized by SAAMI, chamber dimensions, throat length and usable overall cartridge length can also vary between individual rifles.

The 6mm-204 RR performs especially well with bullets weighing from 55 to 80 grains. From the 18½-inch barrel of the custom Sako rifle featured in Handloader, 55-grain bullets approached 3,500 feet per second, 70-grain bullets reached nearly 3,200 feet per second and premium 80-grain hunting bullets exceeded 3,000 feet per second. The 70-grain class provided an especially useful balance of velocity, accuracy and striking energy for varmints and predators. Premium controlled-expansion 80-grain bullets extended the cartridge’s usefulness to deer-sized game where legal and when paired with careful shot placement.

Its performance is particularly notable because it requires only about 65 percent as much powder as the .243 Winchester. The smaller powder charge contributes to reduced muzzle blast, mild recoil and potentially longer barrel life while still producing useful hunting velocities from a relatively short barrel. Accuracy from the test rifle was excellent, with favorable-condition groups measuring around three-eighths of an inch at 100 yards. Many test cartridges were loaded to overall lengths between approximately 2.470 and 2.500 inches, making them too long for the original Sako magazine and requiring the rifle to be operated as a single shot. Despite that limitation, the 6mm-204 RR remains an appealing example of modern wildcat development: simple to form, economical with powder and versatile enough to serve as a highly accurate varmint, predator and light big-game cartridge.

Source: Wolfe Publishing, Handloader Issue No. 344, June 2023.