The .300 Weatherby Magnum is a .30 caliber rifle cartridge created by Roy Weatherby in 1944 and produced by Weatherby. It has become the most popular of all the Weatherby cartridges.
.300 Weatherby Magnum Wikipedia
Roy Weatherby already had experience with other custom cartridges such as his own .270 Weatherby Magnum when he created the .300 Weatherby. Like most of his other magnum cartridges, this is based on a blown-out .300 H&H Magnum case, using the signature Weatherby double-radius shoulder.
Historically, Weatherby claimed that this is the most powerful .30 caliber magnum rifle commercially available, but the recently introduced .300 RUM and Weatherby's own .30-378 Weatherby Magnum are now more powerful. Of course there are quite a few very large .30 caliber wildcat cartridges.
One must note, however, when comparing the .300 RUM and the .300 Weatherby Magnum the difference in factory loadings. Performance data is often listed on the side of the ammunition box for those who wish to do an in-store comparison. On average, Weatherby factory ammo is loaded to higher chamber pressures than Remington or Winchester magnum rounds. The Remington round can be handloaded to equal or even surpass the .300 Weatherby in power.
The .300 Wby is in common use by big-game hunters all over the world.