Definitions for ".17 HM2"
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 case, slightly lengthened and necked down to .17 caliber (4.5 mm) and using a bullet weighing less than half what typical .22 Long Rifle bullets weigh. That ultralight bullet is a key part of achieving the very-high velocity for a rimfire round, but it produces compromised performance on the other end when it hits.