Proposed: The .243 Magnum Rimfire

By Chuck Hawks

If the shoulder of the .17 WSM case were eliminated, creating a straight case, and the resulting case shortened by about 1/32 inch (or even as much as 1mm), the result would be a case suitable for a .243 caliber rimfire cartridge for the Ruger Single Seven (6.5 inch barrel) and SP 101 (4.2 inch barrel) revolvers.

The maximum cartridge overall length should be specified as 1.475 inches, the same as the .327 Federal Magnum. Thus, any revolver with a cylinder long enough to accommodate the .327 Mag. or the .38 Special could also be chambered for the new .243 Magnum Rimfire.

The bullet weight would be 50-60 grains with a truncated cone shaped nose. Such a bullet profile would be well suited for use in revolvers and also for tubular magazine fed rifles.

For example, a .243 caliber, 58 grain bullet would have a .140 sectional density (SD), which is superior to .224 caliber 40 grain (SD .114) and 45 grain (SD .128) bullets and nearly identical to a .224 caliber 50 grain bullet (SD .142).

It is also superior to .251 caliber 50 grain (SD .113) and .257 caliber 60 grain (SD .130) bullets. In fact, the SD of a 58 grain .243 bullet is superior to a .32 caliber (.312 inch) bullet weighing 90 grains (SD .130). This bodes well for the penetration potential of the .243 Magnum Rimfire.

The proposed .243 Magnum Rimfire is not intended for use in short barreled pistols. The powder should be chosen to provide the desired performance from a 6.5 inch to 9.0 inch revolver barrel and the specified test barrel length should be 6.5 inches (vented). To preserve some semblance of the desired ballistics, no handgun with a barrel shorter than four inches should ever be offered in .243 Magnum Rimfire.

Nor should the new cartridge's performance be compromised by powder and bullet selection specifically intended for use in rifles, although the cartridge is likely to be offered in companion rifles, in the same way the .357 Magnum, .44 Magnum and .45 Colt centerfire revolver cartridges have successfully been adapted to lever action carbines. I am confident that a cartridge properly designed for long barreled revolvers will also perform adequately in carbines.

The .243 Magnum Rimfire should be capable of being loaded to about the same muzzle velocity (1400-1500 fps from a 6.5 inch barrel) and trajectory as the .22 WMR and other current magnum revolver cartridges, but hit harder than the .22 WMR, sell for less than any centerfire cartridge and kick much less than the .32 H&R or .327 Federal Magnum centerfire cartridges.

Assuming, for the moment, a 58 grain JHP bullet (BC .151) at 1450 fps MV, the muzzle energy would be 271 ft. lbs. For comparison, this is superior to all .22 WMR, .32 ACP and .380 ACP loads and most .32 H&R Magnum and .38 Special loads. Not bad for a relatively inexpensive rimfire cartridge!

Here are the downrange velocity figures (in feet-per-second) for that load:

  • 1450 fps at muzzle; 1352 fps at 25 yards; 1269 fps at 50 yards; 1195 fps at 75 yards; 1133 fps at 100 yards; 1082 fps at 125 yards; 1038 fps at 150 yards.

Here are the downrange energy figures (in foot-pounds) for that load:

  • 271 ft. lbs. at muzzle; 235 ft. lbs. at 25 yards; 207 ft. lbs. at 50 yards; 184 ft. lbs. at 75 yards; 165 ft. lbs. at 100 yards; 151 ft. lbs. at 125 yards; 139 ft. lbs. at 150 yards.

From a revolver zeroed at 100 yards, the trajectory in inches would look like this (0.75 inch sight height, standard atmospheric conditions):

  • -0.75 in. at muzzle; +1.39 in. at 25 yards; +2.34 in. at 50 yards; +1.93 in. at 75 yards; 0 at 100 yards; -3.63 in. at 125 yards; -9.11 in. at 150 yards.

These trajectory figures are very comparable to the other magnum revolver cartridges, from the 40 grain .22 WMR load to the 300 grain .454 Casull load. (See The Magnum Pistol Cartridges from .22 WMR to .454 Casull for details.)

Of course, these are estimated ballistics. The actual MV for a mass produced .243 rimfire cartridge shooting a bullet weighing 50-60 grains might vary by +/- 50 fps. However, the resulting ballistics would still be much the same.

In a Single Seven with a 6.5 inch or 7.5 inch barrel the .243 Magnum Rimfire would be a fine, mild, handgun hunting cartridge for small game, varmints and small predators (coyotes, etc.). In an SP101 with a 4.2 inch barrel it would make an excellent kit gun and also have personal protection applications.

It should not be news to the folks at Ruger that the .327 Magnum generates unpleasant recoil and muzzle blast in the SP101, especially for beginning handgunners, which includes a lot of female shooters. The .243 Magnum Rimfire should appeal to these shooters, as well as handgun hunters.

As previously mentioned, the .243 Magnum Rimfire could also be adapted to rifles. Any rifle with an action suitable for the .17 WSM should be able to accommodate the .243 Magnum Rimfire. Its performance in long guns would be more of the same good thing, but that is a story for another article.

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Copyright 2016 by Chuck Hawks. All rights reserved.