The .17 Mach II (.17 HM2) is based off the CCI Stinger .22 Long Rifle (.22 LR) shell case that has the neck reduced to fit the .17 caliber. The .17 HM2 is a rimfire cartridge with 17 grain that has a velocity of 2,100 ft/s. The shell casing and bullet are designed to have a higher velocity than other caliber rounds in its class, so it costs more to produce than the .22 LR bullets but is still relatively inexpensive.
Designed in 2004 by CCI, Hornady, and Eley the .17 HM2 didnt receive as much attention as the previous .17 HMR. The .17 HM2 typically requires a modification of barrel change on most bolt action .22 rifles, and is not suitable to be placed in modified semi-automatic .22 rifles due to abnormal pressure changes on blowback they require. The bullet is designed to be lighter to give it the best possible maximum velocity, flatter trajectory, and longer effective range (Up to 160m) for small game hunting.
The .17 HM2 can be used on converted barrels such as the Ruger 10/22, but are harder to acquire due to lack of popularity. The bullet is designed to travel farther for small game hunters to hit targets at longer ranges while keeping the carcass meat salvageable for eating. At closer ranges the energy from impact can cause more damage and damage the majority of a carcass.