I am not sure 45 fpe will make a big difference from 40, but if it were me I would not risk a body shot on a Fox, unless I had a .25 Condor or Sumatra .25 which both get 70-80fpe from the factory, and even then.. I would try really hard for a head shot. But if I had Manny's Evanix .25 auto, then I might shoot a burst in the heart/lung area, or empty the clip on it
The EunJin at 50 FPE would definitely outpenetrate the JSB at 45 FPE.... Bob
Trevor, just out of curiosity how big is this fox really? Your sig location says your in Maine. Are you talking red's ? Most red's up here in my neck of the woods (Upstate NY) are only 6-8pounds. They look pretty good size with winter coat, but under all that hair is a tiny critter. They can get bigger, around 30lbs. at high end. But a 30lb.er will look like a dang ol' monster fox. I havent ever seen any that large. Just asking, cause you might be overthinking what you need. Its easy to do. A lot of folks thinks coyotes are huge, but most of them are under 40lbs. Full coat makes em all look bigger than they are. People think of their house dogs weight and size, and see an equally sized yote and think they would be the same weight. Not so much.
Trevor, Bill put up some real good advice up above with his energy = weight.Like he mentions about hydrostatic shock, firearm (pb) hits on critters at high speed create a Temporary Wound Channel, as well as a Permenant Wound Channel. The perm. one is the path the bellet actually takes cutting thru body. The temp. one is where all the shock trauma, and organ destruction comes from. You will get a temp. wound channel with an AR as well, but no where near as large or destructive. However, with an air rifle shooting 40-45fpe (at target), I'd shoot a 15lb fox any day of the week, twice on Sunday.If you take the heart/lung shot, just treat the after shot routine like you would for deer. Dont jump right up after him. Give him 20 minutes. As long as you dont pressure him, he shoudnt go farther than the nearest cover and drop down if he even makes it that far.