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A cartridge comparison between the .22 ILARCO (two middle cartridges), the .22 Magnum parent cartridge (left), and a .22 Long Rifle round w/ lead round nose bullet (right). Photo by AaronN322 on the Gun & Game forum.

The .22 ILARCO, also referred to as the .22 Winchester Short Magnum Rimfire or .22 American was an experimental rimfire cartridge developed by Winchester in 1987 for the American-180 submachine gun. At the time of development, the American-180 design was taken over by Illinois Arms Company, resulting in the name ".22 ILARCO", but as was noted above, it is known by a number of other names as well. The cartridge never actually caught on, and production totally ceased upon Illinois Arms Company being bought by Feather Industries of Boulder, Colorado.

Development & Failure

Following Illinois Arms Company's purchase for the rights to the American-180, they believed that a more powerful cartridge than the .22 Long Rifle would be a better fit for the gun, especially when marketing to law enforcement agencies and other organizations.

This new cartridge of course had multiple advantages: More energy than typical .22LR ammo, as well as "...the short case used the same jacketed bullet as the parent cartridge and didn’t pick up dirt and debris the way sticky, outside lubricated .22 Long Rifle cartridges can. With a 165-round magazine and full-auto fire, this was a matter of some importance." - Frank Barnes, Cartridges of the World (16th Edition).

Another selling point for converting to this more powerful cartridge is that only the existing barrel would need to be changed. Everything else could be kept original from factory, making conversion easy and cost-effective.

According to "Aaron" in a thread at the Cartridge Collectors Forum from 2010:

"It is basically a hot-loaded .22 Long Rifle round, with the heavier bullet of the .22 Winchester Magnum Rimfire round and a strengthened .22 Long Rifle case with much more propellant. This was done to increase the power of the American-180, which could not chamber the longer .22 Winchester Magnum Rimfire round, without having to redesign the action and magazines. The power is almost the same as that of the .22 Winchester Magnum Rimfire. The .22 ILARCO round never went into large-scale production, and the sale of the American-180 patent ensured this. The .22 ILARCO is now a collector’s item."

In the same thread, Aaron also referenced an email response sent to him by a company handling American-180 parts and accessories:

"The short magnum was a standard Winchester Magnum with the brass length shortened, so that when put back together, it ended up being the same overall length as a standard long rifle cartridge but of course a little fatter. Their was only one run of 1M and only several hundred remain. They were produced in bulk and to my knowledge no boxes were ever made..." - Val Cooper

Ballistic Performance

Frank Barnes notes in his book Cartridges of the World (specifically the 16th edition), that the round utilizes a 40 grain FMJ bullet, an unspecified amount of "FL" powder, and achieves a muzzle velocity of 1.350 feet per second for 160 ft lbs of energy. This is apparently Winchester's factory load. Although the information provided by "Aaron" in the section above claims that this cartridge achieves similar performance as the .22 Magnum, this doesn't appear to be the case. In fact, Winchester's factory loading is actually weaker than modern high-powered .22 Long Rifle loadings such as the CCI Velocitor...

Bullet Weight / Type Powder Type / Amount Velocity Muzzle Energy
40 Gr. FMJ "FL" - Unknown amount 1,350 fps 160 ft lbs


  • Parent case: .22 WMR
  • Bullet diameter: .224 in (5.7 mm)
  • Neck diameter: .242 in (6.1 mm)
  • Base diameter: .242 in (6.1 mm)
  • Rim diameter: .294 in (7.5 mm)
  • Rim thickness: .050 in (1.3 mm)
  • Case length: .669 in (17.0 mm)
  • Overall length: 0.950 in (24.1 mm)
  • Primer type: Rimfire

Alternate Names

The cartridge seems to be referred to by many different names by various sources, so hopefully we've covered most or all of them below:

  • ".22 American" (named for "American Arms", the original company making the American-180)
  • ".22 ILARCO" (for "Illinois Arms Company", who bought American Arms and the American-180 patents)
  • ".22 Short Magnum"
  • ".22 SM" (SM for "Short Magnum")
  • ".22 WMR Short (from Cartridges of the World, 14th Ed.)
  • ".22 Winchester Magnum Rimfire Short"
  • ".22 Winchester Short Magnum Rimfire"