John;

What's the gun?

As for ballistics, it is as you say almost a dead-ringer for the .22 Savage which is identical in performance to the 5.45 Russian and heavy-bullet 5.56/.223. I have no experience with the 6mm but do with the 5.45 Russky and that one kills dogs the size of the small deer just fine.

It'll be a bit rough on meat rabbits and hares for sure but I'd think OK on foxes if you can get some hard bullets for it.

This must be a gun project of yours we need to hear more about...