Troopers say Gregory Dean Jr. of Hopewell Junction was stopped for an inadequate light on his license plate about 9:45 p.m. Sunday in New Lebanon.
They say their investigation showed the semi-automatic on the passenger seat was a legally possessed but its magazine contained nine rounds, not seven as required since April 15.
New York's new measure outlaws magazines that carry more than seven bullets, but seven-bullet magazines aren't made.
Troopers say Dean was charged with aggravated unlicensed driving for a suspended license and unlawful possession of the ammunition feeder, both misdemeanors.
He's due in New Lebanon town court May 23.
A telephone listing for Dean was out of service Monday.