I once saw a cow elk shot at least 800 yards (according to the rangefinder I had with me). The shooter was about 13 years old, and the cow just stood there on the hillside while the shooter kept blasting away with a .308. The cow was hit in the front leg on the 14th shot, and gut shot on the 16th. The cow laid down, head up, until it was put out of it's misery at close range later.

The elk were about 150 yards from me and I was between the shooter and the elk, but apparently neither the shooter or the proud father could see my blaze orange cap and vest even though I was standing completely in the open. The shots came from so far away that the elk were not even spooked until the cow was finally hit. I sat down and waited to meet the jerk who had shot over my head, but when it turned out to be a proud father and child I just offered congratulations and walked off.

From my point of view the event was wildly unethical, but the shooter and proud father probably did not and had a story to tell of great marksmanship by the child (if you leave out a few details). To each their own.

So I guess the answer to the original question is a .308 and a big pile of ammo.

The OP seems to be gone since starting this mess.