The .30-378 Weatherby had been around for decades as a wildcat cartridge based on the large .378 Weatherby Magnum case necked to .30 caliber. In 1998 Weatherby introduced it as a standard industry cartridge and began offering rifles and ammunition. It features the traditional Weatherby double-radius shoulder and has an overall length of 3.648 inches, which requires a large action (such as the Mark V) to house it. Factory loads are advertised to push Nosler 165-grain Ballistic Tip bullets at 3,500 fps, 180-grain bullets at 3,420 fps, or the Nosler 200-grain Partition at 3,160 fps, all of which easily out-run the respected .300 Weatherby Magnum. It has enjoyed some popularity in long-range competition as many load and bullet combinations remain supersonic at 1,000 yards, but it has also gained acceptance hunting big game in open country.

Case capacity can be as high as 120 grains with some propellants, and there are only a few slow burning powders that are suitable to take advantage of its over-bore nature. This performance is not free, as recoil and muzzle report are significant (at least in field weight hunting rifles), and rapid throat erosion will shorten barrel life.

To prevent erratic ignition and hang-fires, do not reduce powder charges below “start” loads and use only large rifle magnum primers with the Federal 215 being employed to develop the accompanying data.

Ammunition that is to be used in a repeating bolt action rifle should have the bullets crimped in place to prevent their being deep-seated when subjected to pounding recoil while in the box magazine.