Years ago I shot a buck and got a similar reaction. He was standing behind a big oak tree and some brush. All I could see in the clear was his upper neck and head. Shot him under the earhole with a 35 Whelen. He hit the ground and his legs were running for a few seconds. He wallowed out a big circle in the leaves and broke one of his antlers off. I was surprised he moved at all where he was shot.
