The .308in./7.65mm former NATO round is similar to the previous US battle round, .30'06 Springfield. These calibers are North American "Big Game" calibers and are reliable up to large bull elk. As a sniper caliber, the .308 was used in VN. It does well in the jungle because the round is heavy and traveling fast so that leaves and twigs won't disturb its path to target.
Other than the AR15/M16 in .223in/5.56 NATO (current), .223 caliber is a "Varmint" caliber. Long shots across flat land with no vegetation to penetrate, to down a coyote or vermin. If you'd lost a good horse to a prairie dog hole you'd want to shoot the "cute" little creatures too.
My guess is the A-hole that is doing the carnage is a disturbed little wimp and is using this caliber because it shoots straight, has a small report, and doesn't kick much.