Q: I’m in the market for a .30 magnum, but can’t decide between the .300 WSM and the .300 Win. Mag. All the reports I’ve read concerning the Winchester Short Magnums seem indicate that shorter and fatter is better, and to endow them with a magical quality that allows them to outperform cases of larger capacity like the .300 Win. Mag. Yet, you have often stated that there’s no substitute for cubic capacity. Like you I don’t believe in pie in the sky or the tooth fairy so how can the .300 WSM outperform the .300 Win. Mag?
A: I own a Winchester Model 70 .300 WSM. It’s a good cartridge that performs as well as one could expect from its volume and has some real mechanical advantages over the longer .300 Win. Mag. – all else being equal. Barrel length may be the same for both rounds, but Expansion Ratio which is computed by dividing the volume of the cartridge case by the volume of the barrel bore is a good measure of efficiency. And it favours the shorter case which could explain the velocity gained by the .300 WSM. To properly compare apples to apples, the maximum chamber pressures of the two magnums are not much different at 64,000 and 65,000 psi. With factory loads the .300 WSM shows a 10 fps advantage, probably due to being loaded to an extra 10,000 psi, but in reloads with bullets weighing 180gn and more the .300 Win. Mag. shows a definite edge. Despite claims to the contrary, the smaller .300 WSM does not outperform the .300 Win. Mag; how could it when the latter has superior case capacity? The .300 WSM’s short, fat powder column, however, does give an indication of better ignition and burn with smaller shot-to-shot and pressure variations in a more practical lightweight, fast- handling short-action rifle.