Probably HMR. HMR is a necked down 22 mag. HM2 is a necked down 22 LR. You can put a 17 barrel on a 22LR rifle (think AR conversion) and shoot those nasty little 17 cal boogers.
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Yesterday (10/30/13), the "in stock" notifications for 17 HM2 Mach 2 started flying from Midway, Brownells, etc. but, alas, who can click that quick? I think they produce this rimfire cartridge once a year. So, everyone keep an eye out and let me know if you see any on the shelves anywhere (Upstate).