.17 Mach 2
.17 HM2
introduced in 2004
.22 Long Rifle case, necked down to .17 caliber (4.5 mm)
17 grains
Bullet weighs less than half the typical .22 Long Rifle bullet

.17 caliber bullet is much more difficult to manufacture than .22 LR
.17 Mach 2 typically cost twice as much .22 Long Rifle ammo

The .17 HM2 was never successful