Wonder how many people will be "packin"?
None.
Part A. It's D.C., and gun owners are law abiding.
Part B. The Aunties are/will make and take every possible opportunity to create/take advantage of ANY possible propaganda incident. A marcher who so much as drops a Kleenex on the ground will be Nat'l headlines as an example of the criminal and lawbreaking character of the typical gun-fiend. Do YOU want to be the posterchild for the gunbanning pestilence?
Part C. the movement of 'our' people is being organized to be nationwide, with on hand logistical and legal support on all main routes of travel. The 2ND March is is like unto Martin Luther King or Ghandi; not Charles Bronson's NYC vigilante or other sterling Whorywould fictions.
Part D. Start thinking about what local institutions will accommodate travelers [VFW, Gun club, etc], what key rest areas can be 24 manned for coffee and snacks, what legal folks will volunteer as trouble shooters, when some Nazi State Police Agency starts shaking down carloads of travlers for 'packin' felony heat'. And etc.
Lastly: Post y'r own suggestions here. Local organization, plenty of it, and well administered from the national perspective is going to be the key volunteer effort for success.