As I was retarded this last weekend and left my 5.56 magazine at home during a range outing, I do have experience going the other way (using a Grendel mag to feed 5.56). It didn't work. Not only is the follower different, but the space between the lips is wider to accommodate the wider case of Grendel. I'm pretty sure you can't use 5.56 mags to feed Grendel, but I may be mistaken.
There a quite a few bullets in 6.5 that would meet your needs for pigs and goats, so that shouldn't be a problem. I would have serious thoughts about taking a moose or brown bear with it though! 8)
Sabre Defense - I have no experience, sorry.