Breechplug
As mentioned above in my other reply, the Primer Hole in the ACCURA's BP it too big, all the primers I have tried in the BP are too loose fitting. Most of the blowback is getting through there I suspect. If the primer hole was smaller Im sure this would eliminate most of the problem.
If you make the primer holder area much smaller you might have a problem getting a spent primer out of the cup.
The primer casing acts much like the brass shell casing in your rifle. As it is shot and pressure is exerted the primer can swell to the walls of the cup to created a seal as does a conventional shell casing. After shooting and with some cooling the primer then should contract back to some where it's orginal size so that it can be easily removed. Problem... aliminum/steel batter cups that you find on on a lot of primers today (less expensive) do not expand as well nor do the contract back done as well or as fast.
One of the reasons that I like the Cheddite and and Rem's are they still have conventional Brass battery cups.
BP you may have experiance this in your trap shooting - Shotgun shells with a true brass base, seem to last longer and work smoother (Winchester AA's or Remington STS's) than the steel bases or steel bases that a plated with a brass color. In my Rem 870 with the 28" factory barrel the steel shotguns shells will actually get stuck unless I really wait for them to cool.
Just thinking off the top of my head....