7mm Winchester Short Magnum (WSM)
Date: Aug 13 2013
The 7mm Winchester Short Magnum (WSM) cartridge was developed by necking down the .300 WSM case to accept 7mm/.284-inch bullets and repositioning the shoulder slightly to prevent the latter cartridge from being inadvertently chambered in a 7mm WSM rifle. Part of its merits include functioning through a .308 Winchester length action, shorter powder columns with low extreme spreads, and duplicating 7mm Remington Magnum velocities with a lower powder charge while producing less barrel wear.
Not all factory loads are equal; however, most 140-grain bullets are advertised at 3,225 fps, 150s at 3,150 to 3,200 fps and 160s at 3,050 fps. For reference, the Winchester Model 70 test gun used here was fitted with a 24-inch barrel and factory load velocities were typically around 30 to 60 fps lower than advertised speeds.
Several powders duplicated or exceeded factory load velocities. For example, the Hornady 139-grain SP with Winchester 760, IMR-4831, Alliant Reloder 19, Vihtavuori N-165 and Accurate Magpro reached over 3,200 fps. Moving up to various 154-grain Hornady bullets, Hodgdon H-4831, IMR-7828 and Vihtavuori N165 gave top velocities and were accurate. Using Hornady 162-grain BTSP bullets, Reloder 22 proved outstanding with 65.0 grains yielding over 2,900 fps and was especially accurate, while H-4831, IMR-7828, Accurate Magpro and Ramshot Magnum also yielded similar or higher velocities.
In spite of being a relatively new cartridge, there has been a rather wide variance in case neck thickness that ranges from .014 to .022 inch; however, most cases measure between .014 to .018 inch. Cases with the thicker necks, combined with a tight chamber, can give greater pressures. As always, never begin with maximum loads. Start with loads that are at least 5 percent below maximum.