The theory is that the std 209 primers are hotter and allow the bullet to jump a bit before the powder ignites causing accuracy to suffer. Again, this is a theory that I read (or heard) somewhere. True or not????????
But I did buy some T7 primers at wally world prior to the last outing with my Genesis. And my groups were tighter. Was it me, the primers or the rifle, or all 3? Who knows.