I'm looking for a toy series from when I was a kid in the mid to late 90s so I can purchase them on eBay. They were plastic cars of all kinds (limos, Humvee, dune buggy's, regular cars). But they had keyholes for an actual size plastic key that when inserted and turned in the keyhole would reveal hidden weapons of all sorts. Some of the weapons (i.e. rocket launchers, etc.) would actually fire little plastic ammunition. These toys were quite the hit with the kids in my school. Thanks.

Are you thinking about the Mattel Hot Wheels Car series?

Most likely the Key Force series, I have the police car kicking around somewhere.