A year ago this New Year's Eve, John Filippidis of Florida was driving south with his family on Interstate 95 when the Maryland Transportation Authority Police pulled over his black Ford Expedition and proceeded to raid it while his twins, wife and daughter looked on — separated in the back seats of different police cruisers.

The officers were searching for Mr. Filippidis' Florida-licensed, palm-size Kel-Tec .38 semi-automatic handgun, which he left at home locked in his safe. (Maryland does not recognize handgun permits issued by other states.)