Good idea.
We should anticipate media spin, though. If we organize a million gun owner march the media will chant FAIL if we get 999,999.
If we organize a Hundred Thousand Gun Owner March and get 250,000, they won’t trash our numbers (they will have to fall back on lame examples of extremism as usual).
Unfortunately, they won't report it either. Right-to-Life has been sending tens of thousands (or more) to DC every January for 30 years, and most newspapers and TV won't even report it; if they do report it, they under-report the numbers. (Of course, if two dozen "pro-choicers" show up on the steps of the Supreme Court, well, that's front-page news....)
No, the only way for this to make an impact is for FAR more than one million to show up.