I use a .17 remington which is far more powerful than a .17 hmr. You are talking about 4200 fps vs. 2200 fps. I would say that using a .17 hmr for fox is somewhat marginal. I have had them run off using a .22 mag which is a much heavier bullet. Yes, it will kill them if hit them. But a poor hit is going to mean a fox that may not be found. I compare it using a .223 for deer. Yes, it is going to kill them if you hit the in the vitals or in the head but a bullet that hits outside of that area may not do the job quick enough. If you hunt at night you want something that is going to kill them on the spot. It is hell to blood trail a fox a night. A 22 hornet is another good round for fox. It gets a bad rap for not being a accurate round but I have found that the Browing Micro hunter with hornady factory ammo is one accurate rifle. Same hole groups at 100 yards.