This appears to mean a member of congress can carry a firearm within the Capitol Building and its associated offices.  However I doubt this applies while the member is traveling to or from his office, for example between his place of residence in Virginia or Maryland, and his office or would apply if the member stopped at a place of business, such as a fast food store, while 
traveling to or from his residence to the US Capitol.  A similar question came up years ago in Fairfax County Va, when a court judge who carried a firearm when on the bench, if it was legal if he stopped at a store while traveling to and from work.  The Virginia State Attorney offered an opinion that it was not legal for the judge to go into a store carrying a concealed weapon, without a concealed weapons permit.  At the time Fairfax County had never issued a concealed weapons permit.  The permit to the judge was the first one issued by Fairfax County.  The answer is for DC to issue permits!