The 17 HMR is based on a 22 mag case necked down to hold a .17 cal bullet and typically shoots a 17 gr bullet at appx. 2550fps while on the other hand the 17 Mach2 is based on a 22 long rifle case necked down to hold a .17 caliber bullet and it's velocity with the same 17 gr bullet is appx 2100fps so to answer your question there is about450fps difference between the two. The HMR ammo is themore expensive of the two.