| United States|
The .17 Hornady Mach 2, or .17 HM2, is a rimfire cartridge introduced in 2004 by the ammunition manufacturer Hornady, following the successful launch in 2002 of the .17 HMR. The .17 HM2 is based on the .22 Long Rifle "Stinger" case, necked down to .17 caliber (4.5 mm) and using a bullet weighing less than half what typical .22 Long Rifle bullets weigh.
.17 HM2 Wikipedia
The ultralight bullet is a key part of achieving the very-high velocity for a rimfire round. It weighs only 17 grains (1.10 g) vs 30-40 grains (1.94-2.59 g) for typical .22 LR bullets. The .17 HM2 may or may not live up to its "Mach 2" name, depending on geographic location and conditions, with velocities out of a rifle of 2100 ft/s (640 m/s). The velocity is nearly double that of a standard .22 LR, which results in a much flatter trajectory out to the 175 yards (160 m) effective range of the cartridge.
Since the .17 HM2 is based on the .22 Long Rifle converting most bolt action firearms chambered in .22 Long Rifle to .17 HM2 requires only a barrel change. The higher pressure makes conversion of semi-automatic firearms more difficult, as virtually all are blowback designs that are sensitive to pressure changes. Kits for the Ruger 10/22 and AR-15 have appeared, and they replace the factory bolt or bolt handle with a heavier one to increase the bolt mass and compensate for the higher pressure.