Winchester officially announced the .17 Winchester Super Magnum cartridge today, and it does appear to be the fastest and most powerful rimfire cartridge ever. Two flavors have been announced so far; a 20-grain bullet reaching 3,000 feet per second, and a 25-grain bullet traveling at 2,600 feet per second. Muzzle energy on the 20-grain loading is rated at 400 foot pounds, with the slower 25-grain loading measuring 375 foot pounds. By comparison a standard 115 grain 9mm Luger round generates around 330 foot pounds of muzzle energy, so this sort of performance from a rimfire is a true breakthrough. Case capacity exceeds any previous rimfire caliber, and the latest generation ballistic tipped bullets will be utilized immediately in factory loadings. Suddenly, it is realistic to score a humane kill on a coyote sized game animal with a rimfire rifle, every time. Accuracy is said to be very good, but the cartridge’s true claim to fame will be its ability to resist wind drift significantly better than .17 HMR or .22 Magnum. Effective range is well beyond 200 yards.

The most interesting thing about the .17WSM may be the casing it uses, which doesn’t come from a firearm at all. Winchester manufactures .27 caliber rimfire casings already for use in powered fasteners (what we rednecks call “nail guns”). What effect this will have on the cost of the new cartridge is unknown, but the pundits are agreed that any new rimfire caliber needs to be affordable or its chances of success are slim.

With these numbers and Winchester’s name on the cartridge, expect multiple manufacturers to offer rifles chambered in .17 Winchester Super Magnum before the end of the year. Will the .17WSM be suitable for semi-automatics? With the chamber pressures involved in firing a 20 grain bullet at 3,000 fps, I would guess that a simple straight blowback design would not suffice. A rifle with a gas operated locking system would be needed. An AR-15 style varmint rifle chambered in this caliber would be a terror to groundhogs.