A 5" gong at 300 yards, in windy conditions, from field positions, is a challenging target. I like that you're out, practicing from field positions.
Keep on practicing, and you're going to be fine. Elk have a bigger kill zone than your 150 pound whitetail. A target I often use for pre-season practice with my hunting rifles is a standard 9" paper plate. When I can keep all my shots from my hunting rifles & field positions, I know that I'm good to go. It's worked for years. Whatever distance I can do that at, is about my max range with that rifle, in the field.
Normally I do a lot of practice at 300 yards, and that's worked just dandy. I've taken a few shots at game beyond 300, but not many.
Regards, Guy
