I've had the same thing happen when I loaded for a friends 7mm mag. He couldn't shoot the load I reloaded for him. Instead of changing the charge or powder he just gave up. Some rifles are very touchy about loads. Most of the time it is trying to push the bullet too fast. Wrong bullet weight is a major factor on some rifles at one end of the twist spectrum usally the slower twist barrel have more problems stabilizing heavier bullets vs light. Anyway don't give up on the reloading part you can find the load that will outshoot any factory load you put through it. But then again 3" groups at 500yds is almost phenomenal. I must say you had a perfect day to shoot that group. Kudo's to you. I always think I'm shooting good to keep them in a paper plate at 500yds