I have not owned nor have I used a 17 hmr, but I think it would be a great round to use on gray's their structure is more rabbit like than other predators. When we do get cold weather and can get a few bucks for them I switch to a 22 mag but otherwise I stick with a 22 lr. I do have some Bobcats here too but the grey fox is far and away the most common predator in my area. I call at night almost exclusively. I will say they learn fast, and when they figure out you are calling them they will torment you. I have had them sit down wind at the edge of my light and yip at me. If they are unwise to a call, they are almost retarded I have had them jump in the cab of a pick-up when using the tape deck in the truck. The best advice I can give is do not call one up and not kill it. I have had them as pets before and they act more like nervous house cats than dogs. Also cut the squeaker out of a dog toy that works well to get them to pose for a shot. Good luck hunting gray's is addicting especially if your first experience is hunting some that are not wise to being called. If you are not going to or it is not legal to hunt them at night I will not be much help.