The .300 Weatherby Magnum is a .30

7 mm caliber

This article lists firearm cartridges which have a bullet in the to caliber range.*Length refers to the cartridge case length.*OAL refers to the overall length of the cartridge....

 caliber

Caliber

In guns including firearms, caliber or calibre is the approximate internal diameter of the barrel in relation to the diameter of the projectile used in it....

 rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

 cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 created by Roy Weatherby

Roy Weatherby

Roy E. Weatherby was the founder and owner of Weatherby, Inc., an American rifle, shotgun and cartridge manufacturing company set up in 1945. Weatherby created an entire line of custom cartridges, and was one of the people responsible for the industry interest in high-speed cartridges...

 in 1944 and produced by Weatherby

Weatherby

Weatherby, Inc. is an American gun manufacturer founded in 1945 by Roy Weatherby. The company is best known for its high-powered magnum cartridges, such as the .257 Weatherby Magnum and the .460 Weatherby Magnum...

. It has become the most popular of all the Weatherby cartridges.
Roy Weatherby already had experience with other custom cartridges such as his own .270 Weatherby Magnum

.270 Weatherby Magnum

The .270 Weatherby Magnum was the first belted magnum based on the .300 H&H Magnum to be developed by Roy Weatherby. It has the characteristic double-radius shoulders and is necked down to accommodate the .277in bullets. Being a proprietary cartridge, the .270 Weatherby has no official SAAMI...

 when he created the .300 Weatherby. Like most of his other magnum cartridges, this is based on a blown-out .300 H&H Magnum

.300 H&H Magnum

The .300 H&H Magnum Cartridge was introduced by the British company Holland & Holland as the Super-Thirty in June, 1925. The case was belted like the .375 H&H Magnum, and is based on the same case, as also is the .244 H&H Magnum. The belt is for headspace as the cases' shoulders have a narrow...

 case, using the signature Weatherby double-radius shoulder.
Weatherby claims that this is the most powerful .30 caliber magnum rifle commercially available, but the recently introduced .300 RUM

.300 Remington Ultra Magnum

The .300 Remington Ultra Magnum, also known as the .300 Ultra Mag or .300 RUM is a 7.62 mm caliber rifle cartridge , 7.62x72mm, or .30 caliber rifle cartridge introduced by Remington Arms in 1999. The .300 Remington Ultra Magnum is one of the largest commercially available .30 caliber magnums...

 is now more powerful.