300 Winchester Short Magnum (also known as .300 WSM, commonly pronounced as "Whizzum") is a .308 caliber short magnum cartridge that was introduced in 2001 by Winchester. The cartridge overall length is 72.64mm, cartridge case is 53.34mm in length and the bullet diameter is 7.62 mm (common to all U.S. .30 calibre cartridges). The principle at work in the short magnum cartridge is the advantage of fitting larger volumes of powder in closer proximity to the primer's flash hole, resulting in more uniform, consistent ignition. In field use, this round mirrors the performance of its older counterpart, the .300 winchester magnum, which is based on a modified .375 holland and holland belted magnum casing.
The advantages of the newer, shorter round are limited, but worth considering. Most notably, the felt recoil of these rifles is less sharp than the longer, narrower magnums. The wide chamber allows gasses re-entering the bore after the departure of the projectile to decompress slightly before slamming into the inner base of the casing, softening the felt impact. Less important to most rifle shooters is the increase in efficiency of shorter casings, which results in greater inherent accuracy.
When addressing this topic, it is useful to consider the true degree of accuracy involved. This degree is fairly minute for most uses, and is primarily important to target shooters, and hunters who rely on a rifle to be capable of extreme long range accuracy. Very few rifle shooters are able to shoot better than their rifles, and as such, this aspect is largely academic. The basic issue at hand is that this cartridge is in a class that is already filled with numerous choices in similarly performing rounds, and has little to distinguish it but nominal increase in accuracy, and lighter felt recoil. The .300 WSM is, however, the most modern .30 caliber cartridge in the magnum class, utilizing the best available knowledge in cartridge design. As such it represents the best combination of ballistic performance, round-to-round consistency, and reduced recoil and muzzle blast.
300 WSM is used in the Western United States for elk, mule deer, and whitetail and on the plains, where long range shooting is a must. Also, although relatively new, the 300 WSM has already had some success in benchrest shooting. As with all high speed large game rounds, bullet construction plays a major role in terminal ballistic performance, and is far too complex to enter in this space.