There have been several different .17 caliber rifles- rimfire and centerfire. Right now, I am guessing that you mean the two most common .17 rimfires- the ,17 Mach 2, and the .17 HMR. The Mach 2 is basically a .22 Long rifle necked down to .17. The 17 HMR (Hornady Magnum Rimfire) is a .22 Magnum necked down to .17. It has more energy than the .17M2. They two cartridges DO NOT interchange. Ammo is more common for the .17 HMR. No experience with the M2, but the HMR is incredibly accurate at 100 yards.
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