In 1944, Roy Weatherby introduced the 300 Weatherby Magnum, his most popular chambering. It uses a slightly shortened version of the case used for the 300 H&H, with reduced body taper, an odd (and particularly inefficient — in terms of ballistic efficiency, it comes close to being the worst possible shoulder design) double-radius shoulder, and a long neck. It offered impressive performance as loaded, at first, by Weatherby and then by Norma. As a proprietary cartridge until quite recently, pressure was never standardized and it is fair to note that ammunition for the 300 Weatherby was routinely loaded at higher pressure than ammo for any other 300 Magnum was. Obviously, this gave the 300 Weatherby a performance advantage with factory ammo and handloads that fell within the safe pressure levels for which the guns had been proof tested. After standardization under SAAMI, much of the original velocity advantage offered by the 300 Weatherby vaporized with loads at what are realistically saner pressure levels. One problem with loads at higher pressures than are normally used for SAAMI- and CIP-standardized chamberings is that properly proof-testing the guns requires loads that generate so much pressure the proof-testing process is difficult and significantly more expensive because case head failures and stuck cases are common. An issue with all original Weatherby chamberings resulted from Weatherby’s desire to make his designs work with the slowest available propellant at the time, which was IMR 4350. To do this, the only way he could make his designs work well with a near case full of propellant was to include a significant freebore in the chamber design. Many gun-writers have claimed he freebored his chambers because he believed this resulted in higher velocity with loads using more of the same propellant to generate the same peak pressure. I am unconvinced. I suspect Roy Weatherby was smart enough to test this and, when he did test it, he would have instantly discovered free boring added nothing to velocity but required the use of more propellant to get the same velocity. However, he was also enough of a salesman to work his freebore design to his advantage and it did allow him to use a charge that better filled the case and likely gave better accuracy because that reduced the charge-position effect (see a discussion of that in the 45 Auto Rim text). As with the 257 and 270 Weatherby Magnums, and other, later, original Weatherby developments, Roy Weatherby worked tirelessly to promote his rifles and his chamberings. At this, he was extremely successful. As was a host of standard belted–magnum cartridges, this case was derived from the circa 1912, 375 H&H.