The .303 Savage cartridge is a rimmed, .30 caliber riflecartridge developed by the Savage Arms Company in 1894 which was designed as a short action cartridge for their popular Savage Model 1899 and 99 hammerless lever action rifle. The cartridge was designed for smokeless gunpowder 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. 
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, but not as successfully in other rifles. However, the pointed-tip bullets gave it a ballistic advantage over other cartridges such as the .30-30. 
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. Reloading ammunition and reformimg brass cases should not be undertaken by anyone unfamiliar with the subject.
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 (or .303 British cartridges in a .303 Savage weapon) is guaranteed to have serious consequences. Such attempts will severely damage the firearm and possibly injure the user.