it's a felony for a citizen to possess firearms or any type of weapon on the premises of any government building in DC without having the necessary licenses and credentials to do so.
DC has some strict gun laws and usually you're only supposed to keep the guns in your house, and they must be unloaded and secured seperately from your ammunition when transporting. you can't carry them open or concealed on government property. once you go west of the potomac and into virginia the laws are much more relaxed. but tasers, bear spray, firearms, or anything like that is generally prohibited from most areas in DC, and you're better off just leaving them at home.
code.dccouncil.gov
it's up to the judges discretion as there is no mandatory minimum sentence but the first offense is a penalty of up to a $5000 fine and 5 years in prison unless you're a convicted felon possessing a gun and then you're up shits creek with all kinds of new charges and penalties. the sentence is doubled to up to $10,000 in fines and 10 years in prison if it's not your first offense. and doubled again if you were carrying on the grounds of a gun-free zone, which would include all government buildings.