You might also have to play with your bench technique, and not just play with the loads. I've had a couple of rifles that would change group size significantly if you changed your "hold" in the bags. One in particular I had to rest my off hand on top of the scope, instead of holding the stock. Some may like to be "free recoiled", and some want to be held down firmly both fore and aft.

Have you checked your bedding, and your action screw tension? Is the tang area floated? Try moderate to firm tension on the front action screw, and very little, but just snug tension on the rear action screw and see if the group size changes. If you watch the barreled action in the stock as you loosen either one or both action screws, do you see it "jump" in the stock?

You may have to diddle around for a while to find "your load" for that rifle. It happens. My Stevens 223 will cut .4's center to center with my handloads, but I couldn't get any of the factory ammo I tried to shoot 1" or less.

My 243 likes W-760 and 87 gr Vmax bullets.

Have you played with seating depth?