Originally Posted by Mikecr
That's a really big difference. Too much for powder alone.
I would definitely try it again
+1, I agree. Are these your first handloads through the rifle or your first significant shooting of the rifle? I ask, because I had a similar issue in the past. The problem turned out to be, well, me. I have found that each new rifle has a bit of nuance in how it likes to be shot. Anymore, I'll load and shoot a box or two of a random load just to get a feel for what is most consistent on terms of form/set-up/trigger pull. Then, when I go to ladder test powder weights, I'll shoot 3 or 4 groups first just to get settled in. That has proven to be a good system for me.
Please note that I am in no way trying to be offensive or cast doubt on your ability. Just sharing my experience in case it helps you. I hate the randomness of these forums as I am certainly not looking to insult someone who just took third place at Camp Perry...
Ps I am a big Cooper fan and have several that shoot really well. I hope you enjoy yours!