This 1994 chambering is based upon an entirely original case that features a .580-inch head size. The rim is essentially identical to the .378 Weatherby case, but the design has not belt. Therefore, case capacity is substantially greater for any given cartridge length. This standard rimless bottleneck design also facilitates proper chambering, with tight tolerances. Design intent was the acceleration of 180-grain hunting bullets to a velocity in excess of 3500 fps from a 26-inch barrel, without exceeding about 62,000 psi (piezo transducer pressure units) - a typical modern cartridge pressure limit. The .300 Pegasus easily achieves this goal.

Rifles originally chambered for the .378 and .460 Weatherby numbers can be rebarreled to accept this cartridge.

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