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17 HMR is a necked down .22 Magnum used to achieve higher velocity/flatter trajectory with a smaller, lighter bullet. 17 HM2 is the same concept but based on 22 LR instead.

Michael, The 17HMR is the Magnum round and the Mach 2 the standard round. (Same difference as 22 Magnum and 22LR). Picture difference here : https://en.wikipedia.org/wiki/.17_HM2