.17 HM2
The .17 Hornady Mach 2, commonly known as the .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 the weight of a typical .22 Long Rifle bullet. == Performance == The weight of the bullet is a key part of achieving a very high velocity for a rimfire round.