Take a loaded 9mm magazine and chamber a round. Then remove the magazine and look at the top round. It's usually pulled forward a bit and can strike the slide lock actuator in the next couple of rounds. Just checking with the rounds all the way to the rear doesn't cover this. The 9mm is really prone to this and especially with the 7-round magazine.
Wynn
USAF Retired '88, NRA Life Member. Wife USAF Retired '96
Avatar: Wynn re-enlists his wife Desiree, circa 1988 Loring AFB, ME. 42nd BMW, Heavy (SAC) B-52G's
Frédéric Bastiat’s essay, The Law: http://mises.org/books/thelaw.pdf
Thomas Jefferson said
“A government big enough to give you everything you want, is strong enough to take everything you have.”
and
"Peace is that brief glorious moment in history when everybody stands around reloading".