It's hard to believe but the kind of primer can make a difference in accuracy. Whether it is pushing something off the load, I would not know. Since your shooting loose powder, I hardly doubt that as the extra heat from the Cheddite primer would simply flood the charge with flame and should cause a good solid ignition.

The reason your 240 might not shoot as well could be simply your rifle does not like that weight of bullet. Try dropping the powder charge down to 80 grains next time and see what happens. Although if you're getting that kind of accuracy with the 300 grain. That is where I would stay.

Sounds like a great rifle, and an excellent report there... nice shooting.