Obviously something changed. Think back through your last 3 or 4 times to Range. Did you do or use anything different? Temperature? Don't rule out your scope just because your screws are tight. I had one go bad on my 3006 that drove me nuts because it wouldn't group. I almost got rid of a perfectly good gun and decided to try a different scope before selling it. The cross hairs were moving every time I shot. Now I will never get rid of that gun. What scope do you have on it?