The .17 Mach 2, also known as .17 HM2, is a rimfire cartridge introduced in 2004 by Hornady. It is based on the .22 Long Rifle case, necked down to accept a .17 caliber projectile. The round offers higher velocities than .22 LR, making it ideal for small game hunting and varmint control. It provides flatter trajectories and greater accuracy than traditional .22 rimfire cartridges.
The .17 Mach 2 is available in a range of rimfire rifles, particularly bolt-action and semi-automatic models designed for varmint hunting. Conversion kits are also available for some .22 LR firearms. This conversion process typically only involves swapping a barrel on bolt action .22 LR firearms. The process of converting a semi-automatic rifle is more involved and usually requires the swapping of a bolt or bolt handle.