The .204 Ruger is a centerfire rifle cartridge developed by Hornady and Ruger. At the time of its introduction in 2004, the .204 Ruger was the highest velocity commercially produced ammunition, and the only cartridge produced commercially for bullets of .204 inch/5 mm caliber.
The 204 Ruger was developed from the .222 Remington Magnum, which has the second largest case capacity in the family that began with the .222 Remington. Only the European 5.6×50mm Magnum is larger, which itself is a lengthened version of the 222 Remington Magnum. The 222 Remington Magnum provides about 5% more usable (below the neck) case capacity than the most popular member of the family, the NATO 5.56×45mm (.223 Remington). To make the 204 Ruger, the 222 Remington Magnum case was necked down to .204 inches (5 mm) and shoulder moved forward and angle increased to 30 degrees. Bullets available in .204 caliber range from 26 to 55 grains (1.7 to 3.25g). The Hornady factory load is listed at 4,225 ft/s (1288 m/s) with a 32-grain (2.1 g) bullet. To achieve these velocities, the factory uses a proprietary powder composition known internally as SMP746, specially formulated by Primex, and not currently (2013) available to handloaders. The propellant features a de-coppering agent that helps prevent fouling. Reloading data from Hornady, using commercially available powders, indicate velocity peaking at just under 4,200 ft/s (1,300 m/s) with the 32-grain (2.1 g) bullet in longer barrels. Many AR-15 rifle manufacturers now offer the .204 Ruger as an alternative chambering alongside the usual 5.56×45mm/.223 Rem.