If your Cooper is a 1/12 twist, just forget using 40 grain bullets, a waste of money.
Stick with the 39 grain, especially the Sierra BK's, they will work just fine, even on fur as compared to any 40 grain bullet.
When you need too, best to re-barrel with a 1/11 twist, then you can go to heavier bullets.
But the 32 & 39 grain bullets work the best in the 20's 95% of the time anyway.
You want so shot heavier bullets go to a larger caliber like 224 or 6mm.
