How old is your breech plug? Is the bottom of the primer pocket nice and square, or is it eroded any around the flame channel from any gas cutting? What is the diameter of your flash hole? Breech plugs are wear items, and need to be replaced periodically.
Your headspace tolerance may be too great for those primers which generally average around .297"-.298". You could try some Winchester W209's, they have measured between .300" and .305" for years, however my last purchase they measured between .297" and .301".
If you load just a primer in your rifle with no powder/bullet load, push it in the primer pocket as far as it will go, then lay a straight edge across the breech end of your barrel. You will be able to see if your primer has any space between it and the straight edge. If you have a set of calipers, measure the primer length, then go to a safe area and shoot just the primer with no load in the rifle, then remove and remeasure the primer length. The primer will slide back in the battery cup, and tell you exactly what your total primer headspace is. If you can get primers closer to that length, it will reduce or eliminate blowback.
A new breech plug could cure your problems as well, and it never hurts to have a spare on hand. Should be able to buy a Encore breech plug for $15-$20, unless you have the Pro-Hunter or Endeavor?