While pheasant hunting last year with a good friend his dog had something similar happen as well. It wasn’t as devastating as your experience but we believe it was barbed wire. We didn’t see the 6 to 8 inch wound until later that evening after returning from the hunt. Hope your dog fully recovers.
Whatever you are most comfortable shooting. I have used my 6.5 Creedmor the last couple years and have harvested a mule deer, buck pronghorn and cow elk.
The saying goes "Beware the man with one gun, he knows how to use it."