are you sure you meant 1/4 inch groups? You're right, that is phenominal.

If the Great Plains Conical is flat based you could try a wonder wad under it and see if that does not give you more consistant groups. It has been so long since I shot Great Plains Conicals, I forgot if the base is hollow or not.

Something else to try since you live in Washington and it is my understanding that you can not shoot sabots when hunting (is that correct?) is you could try some .503 460 grain No Excuses Conicals. I shoot them out of my Wolverine with 85 grains of Triple Se7en or Goex 3f with outstanding accuracy.

Another conical that many shoot out of Knights are powerbelts. I have not tried to many of them in my Knight other then fooling around a little, but some people I talked to got outstanding accuracy with the powerbelts...

Good luck with the rifle.