I am not a guru 2How but I'll offer my .02. 6.8 seems like a good solid hunting round and more than enough for deer and SD. Downside is that its currently a boutique round, expensive, and hard to find in an SHTF situation. .243 Win would be my choice, but I don't know about mags. The same is true with an AR in .308. Its not really an AR as you need a new lower as well. At that point, I'd spring for an M1A, or an FN or a G3 so you get cheap surplus mags and aren't dependent on proprietary mags. Frankly, I've made my piece with .556. Its a paper puncher and SD gun, and a hunting rifle only in extremis. If you want it all, I'd say either go with one of the .308s or buy an AK, depending on your perceived accuracy requirements and wallet capacity. Hope this helps.
FQ13