Range Report Went to the range today and left with an issue.

I have a 10x10in piece of leftover granite I set rounds on to check primers are flush at least.
Anything that wiggles is found quickly.
Old school guy told me to use glass but there is always something getting dropped knocked over on the bench.
 
Brand of primers or touching primers with oily hand can cause that ornot fully seating primer. But every know and again ive had a few do that one on a huge 5x5 bulls