.17 HM2

• Name —.17 Hornady Mach 2 • Origin — United States • Type — Rifle • Designer — Hornady • Design Date — 2004 • Manufacturer — CCI/Hornady/Eley • Production Date — 2004–present • Parent —.22 LR Stinger • Case Type — Rimmed, bottleneck • Bullet —.172 • Land —.168 • Neck —.180 • Shoulder —.226 • Base —.226 • Rim Dia —.275 • Rim Thick —.043 • Case Length —.714 • Length — 1.00 • Primer — Rimfire • Max Pressure — 24000 • Bw1 — 17 • Btype1 — V-Max • Vel1 — 2100 • En1 — 166 • Balsrc — Hornady

The weight of the bullet is a key part of achieving a very high velocity for a rimfire round. It weighs only 17 grains (1.10 g) vs 30-40 grains (1.94-2.59 g) of a typical.22 LR bullet. 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 2,100 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 its 175-yard (160 m) effective range.

Since the.17 HM2 is based on the.22 LR, converting most bolt action firearms chambered in.22 LR to.17 HM2 requires only a barrel change. The higher chamber pressure makes conversion of semi-automatic firearms more difficult, as virtually all are blowback designs that are sensitive to pressure changes. Conversion kits 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 chamber pressure.