The 5mm Remington Rimfire Magnum was based on an entirely new case designed, I believe, in 1969. It resembled a .22 WMR case necked-down to accept 5mm/.20 caliber bullets, but was dimensionally unique. The 5mm Rem. Mag. had a rim diameter of .325", rim thickness of .05", base diameter of .259", shoulder diameter of .259", and neck diameter of .225". Case length was 1.02" and actual bullet diameter was .2045". Factory ballistics called for a 38 grain JHP bullet at a MV of 2100 fps from a rifle barrel, making it the fastest rimfire cartridge in the world at the time.

It was introduced in 1970 in a pair of Remington bolt action rifles, the Models 591 (clip magazine) and 592 (tubular magazine) to a shooting world that could not have been less interested, and it became the shortest lived cartridge introduced since the end of WW II. The Remington 5mm rifles lasted only 5 years (1970-1974), and the cartridge quietly disappeared from the loading list a few years later.

Thompson/Center briefly offered their Contender pistol in 5mm. As far as I know there were no other firearms chambered for the 5mm Magnum, and no other ammunition manufacturer offered the cartridge.

None of that seemed strange to me at the time, as I had predicted the 5mm's demise as soon as I heard about it. The .22 WMR was then loaded with a 40 grain JHP bullet at a MV of 2000 fps. 100 fps more velocity did not seem worth the trade-off of a smaller diameter, lighter bullet, particularly in the case of a weird caliber such as .20, for which there was no previous demand and no accessories.

Today, however, the run away success of the .17 HMR makes me wonder. After all, it is also a sub-.22 caliber rimfire magnum cartridge. Maybe the 5mm Remington Magnum was just misunderstood, a cartridge ahead of its time. Remington has something of a history in that regard. Evidence the centerfire .350 and 6.5mm short magnums (35 years ahead of their time), and .17 Remington (30 years ahead of its time).

The 5mm Rimfire offered more energy down range than the .22 WMR or the later .17 HMR (217 ft. lbs. for the 5mm vs. 170 ft. lbs. for the .22 WMR and 136 ft. lbs. for the .17 HMR at 100 yards). Its smaller diameter bullet was superior in ballistic coefficient and sectional density to the 40 grain .22 WMR bullet and that, plus its increased velocity, gave it a flatter trajectory and about a 25 yard advantage in effective range over the .22 Mag. During its brief life the 5mm Magnum developed a reputation as an effective and deadly varmint cartridge.

For those that own 5mm Rem. Rimfire Mag. rifles, all is not quite lost. Centurion Ordinance has introduced a new factory load for the 5mm. This uses a 30 grain JHP varmint bullet at a MV of 2300 fps with 352 ft. lbs. of muzzle energy. You can read about it at www.aguilaammo.com

I have never owned a 5mm Remington rifle. However, the .17 HMR made me revise my opinion about the capability and usefulness of sub-.22 caliber cartridges and the 5mm Remington Rimfire Magnum was sort of the conceptual Granddaddy of the .17 HMR.