I think you will find that they don't expand, not designed to. They may 'pencil hole' through, and the bullet will carry on out the other side, taking most of its energy with it. Although enough damage will likely be caused by the permanent wound track, death may not be instant; worse of all you could have a wounded fox limping away to suffer badly.
I would swap them for something more suitable myself.
Mate of mine tried some Match Ammo with 'hollow points' the hollow point was tiny, and I believe was part of the manufacturing process, NOT to provide any large degree of expansion.
Hammo