The Museum of the United States Navy is about a mile or so from the US Capitol building in the Washington Navy Yard, DC. It is on at corner of 9th and "M" street in southeast DC. It has one of the best cannon collections in the US, maybe second only to West Point. However after 9-11 West Point's outdoor cannons (most of them) became inaccessible to the general public. The Navy Museum's are still accessible, fortunately.
There are about 15 or 20 outstanding old pieces at Fort Leslie J. McNair, only about 1 mile from the Navy Yard. If you have some business on base, such as eating lunch at the club (open to anyone,) you can get in.