Primers are kind of two-piece. There's a little piece inside, under the cap, called the "anvil". It's what the firing pin crushes the priming compound against. No firing pin/bolt to block it, the cap can blow back out. Plus fragment. Beautiful little slivers of metal to pick out of your eye, etc. Touchy little cusses, too. Why they make primer holders.
Famous "Texan's last words" - "Hold my beer. Watch this!"