I have an upper receiver to use and I was going to order a 6.8 barrel but I am curious is their another caliber that would be better? It will be used for deer/antelope/hogs. Is their another choice that would have more locally available ammo, maybe surplus ammo available? I have had a 6.8 and liked it but I had to order ammo but other then that I am not familiar with any other calibers other then 5.56. What about a 300 whisper?