with those popped out primers what kind of ammo were you using? If you are handloading, some police depts do that, then you might want to get with us on here about it. A bunch of us here are handloaders for the M-14. was it doing it on just one rifle or all of them. one rifle would lead me to a headspace problem maybe but more than one probably ammo. Give us some more details. I don't see where tapping the mags would have an effect on blown primers. We always hit the mag on our helmet a couple times in the service with M-16 and M-14 rifles but it wasn't for that. Sometimes a bullet tip would get caught on the front of the mag and jam. sometimes you get mud or seawater and sand in my case in it and it will tap a lot of it out if you have to use it in a hurry. It was just a habbit everyone has. one police department down here I heard was having problems with commercialy remanufactured ammo. I hope that is all your problem is but let us know.