I had a talk with a long-time CCI employee about the mil-spec primers a year or so ago. It turns out they don't have thicker cups, as many people believe, or even "lower" anvils. Instead they have more rounded anvils, which helps prevent slam-fires when primers are seated slightly "proud" of the case-head.
The biggie is making sure any of them are seated deeply enough, but I have run thousands of BR-4's and 450's in various AR-15's with zero slam-fires.
