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The .303 Savage is a rimmed, .30 caliber rifle cartridge developed by the Savage Arms Company in 1894 which was designed as a short action cartridge for their popular Savage Model 99 hammerless lever-action rifle. The cartridge was designed for smokeless powder at a time when black-powder cartridges were still popular. The .303 Savage round was ballistically superior to the .30-30 Winchester, but only marginally. The .303 Savage remained popular through the 1930s.
.303 Savage Wikipedia
Savage Arms created the .303 Savage as part of an unsuccessful attempt at creating a cartridge for the military. Although the cartridge was never popular with the military, it did become a popular round for civilian hunters. Being a pointed-tip rimmed cartridge, it worked well in the Model 99 rifles that Savage produced because of their rotary magazine. However it wasn't as successful in other lever-action rifles because of their tubular magazines. However, the pointed-tip bullets gave it a ballistic advantage over other traditional lever-action cartridges such as the .30-30 Winchester.
The .303 Savage has a small, but loyal fraternity of shooters who reload this cartridge. While major ammunition manufacturers have long since halted production of ammo, dedicated followers can procure loaded ammunition and brass cases through smaller enterprises. The brass cases can be formed from other more popular cases, most notable the .303 British service round. .30-40 Krag brass cases can be formed into .303 Savage dimensions as well.
The .220 Swift cartridge can be made into a passable .303 Savage cartridge, as they are very close in most dimensions, and after forming the .220 Swift is easily cut to proper length.
As with any firearm, it is essential to use the correct type of ammunition. The .303 Savage and the .303 British cartridge are not interchangeable with each other. Neither the bullet diameter nor the cartridge dimensions are compatible. Attempting to use .303 Savage ammunition in a firearm chambered for .303 British may be unpleasant, but since the bullet diameter of the Savage is 0.308 and the British 0.311, there shouldn't be too much of a problem. The converse is not true. Attempting to chamber a .303 British cartridge in a .303 Savage weapon is guaranteed to have serious consequences. Such attempts will severely damage the firearm and possibly injure the user.