Too much potential for disaster here.  What will probably happen (assuming no incident is provoked) is that they will get to the bridge into DC and be looking at a bunch of guys dressed in full military gear with rifles pointed at them.  Then the march would, most likely, return to Arlington, disband, and go home making the point that armed citizens are not allowed in the nation's capital.  OTOH,  the disaster, and also a likely outcome, would be a shot fired into the marchers, followed by a few round of full auto and lots of casualties.  And all the police would say that they were "afraid for their lives."  It would be a bloody event and reignite the entire gun issue.  If you were the bad guys think how many ways you could plot and turn this little event to your legislative advantage to get what you want.  The one, and probably only, battle THEY are prepared to fight is the one you want to give them.  Not smart. Tsung Sue would not be pleased.

I disagree with the above comment, I'm glad you weren't giving advise to the colonialist rebels in Lexington and Concord. If they took your advise we still be British.