Bolt-action rifles from Kimber, Anschutz, and others came first simply because converting a bolt gun took no more effort than decreasing the size of the hole in its barrel. Blowback-operated autoloaders took a bit longer after it was revealed to the industry by CCI/Speer that time-to-peak chamber pressure for the .17 HM2 differs from the .22 Long Rifle.

In addition to causing bolt-cycle speed of an autoloader designed for the .22 LR to increase beyond an acceptable level, it could also cause the bolt to open prematurely, allowing a fired case to start moving from the chamber before pressure had dropped off. In other words, converting a semiautomatic in .22 LR to .17 HM2 required more than barrel replacement. The problem has been solved by either increasing the weight of the bolt directly or by indirectly adding weight to the bolt by linking it to a reciprocating weight hanging from the barrel and located inside the forearm, as Thompson/Center has done with the R55 rifle.

The .17 HM2 is the .22 LR Stinger case necked down for a .172-inch bullet weighing 17 grains and with a ballistic coefficient of .125. The Stinger was introduced by CCI in 1977, and its muzzle velocity of 1640 fps with a 32-grain bullet quickly added the word "hypervelocity" to the vocabularies of .22 Rimfire shooters around the world. Maximum overall lengths of the regular and Stinger versions of the .22 LR cartridge are the same at 1 inch. Powder capacity of the Stinger is greater due not only to its 0.100-inch longer case but also because it's shorter 32-grain bullet does not displace as much area within the powder cavity as the standard 40-grain bullet of the regular .22 LR. The increase in net case capacity of the Stinger allows the use of a relatively heavy charge of slow-burning powder for a muzzle-velocity increase of about 30 percent over the standard .22 LR. Yet chamber pressure is the same as for the high-velocity loading of that cartridge.

Use of the Stinger case in developing the .17 HM2 enabled engineers at Hornady to safely push a 17-grain bullet beyond 2000 fps from a 24-inch barrel without exceeding the maximum chamber pressure level of the .22 LR. The velocity they achieved is 150 to 200 fps faster than the .17 Aguila, which is on the standard-length .22 LR case. Overall length of the .17 HM2 is the same as for the .22 LR, and that along with the fact that it works fine in most magazines originally designed for the .22 LR helped to pave the way for manufacturers of repeating rifles to quickly adopt the new chambering.

All loads presently available are loaded with the Hornady 17-grain