.300 Pegasus is an American rifle cartridge which was developed in 1994.^[1] It was a completely new case design with no parent case, but has approximately the same breechface diameter (14.73 mm; 0.580 in)^[2] as the .378 Weatherby Magnum, but without the magnum belt. Due to this, the .300 Pegasus has a larger case capacity than the .30-378 Weatherby Magnum.

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The cartridge has an impressively high velocity which makes it quite flat shooting, but estimated barrel life time is short,^[3] with some estimates at around only 500 rounds.^{[citation needed]} The cartridge is usually only recommended for very experienced reloaders.^{[citation needed]}