Christopher Shelton Timmons, 27, of Orange, Va., was charged with carrying a deadly weapon and possessing unregistered firearms and ammunition, said Capitol Police spokeswoman Sgt. Kimberly Schneider.
Timmons was detained after he stopped to ask a police officer for directions and the officer saw a rifle case in the car.
A search of the car, a Jeep Cherokee, turned up a rifle, a live grenade, a pistol, ammunition, magazines with ammunition in them and several other "items of concern," Schneider said.
Police closed nearby streets for several hours while conducting the search. The incident occurred near the Library of Congress southeast of the Capitol.
The FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and District of Columbia police assisted in the investigation.