Strange MOA change

[carrollshooter]

I have a strange problem. My Cooper 21 in 204 Ruger is a 5/8 MOA 5 shot rifle at 100 and 200 yards. When I shoot at 300 the groups are frequently 3 MOA and about 1 MOA to the right. This happens at the same shooting session from the same bench in ideal conditions. I am shooting Sierra 32 gr. BK at 4064 fps. Where do I look for the problem?

 

[CJ6]

Are the bullets going in straight (target) ? What about Wind, Mirage ?

 

[itchyTF]

Twist?
Damaged crown?

 

[Larryh128]

My best GUESS is that for starters, your scope is slightly canted which would explain the off center hits. It doesn't need to be much to give you an inch at 300. Unless you have a scope on the high side of 10x, it could be (?) that you're doing as well as you can for your magnification at 300 yds. It's hard to hold a 9-10x a lot tighter than that at 300.