You might try to find a better fitting brand of primer. I know that primers are not all the same length. Maybe a longer (or shorter) primer would cut down on the blowback.

Also, you did not say whether you are swabbing between shots. I use a patch soaked in a 50/50 mix of rubbing alcohol and windex. After a shot, run a wet patch down, reverse it and run it down again, then follow with a dry patch. Then pop a primer. Then you're ready to load and shoot.