By your own statistics, and assuming we’re discounting independents, Republicans have over twice the gun owners than democrats, putting them over 2/3 of American gun owners. Of course, that’s assuming they are actually “roughly equally sized”. I’d say over double is in fact a massive difference.

You’re right, however, that Democratic gun owners tend to he less vocal about the second amendment. There are plenty of Democrats that own 1 or 2 small firearms for self defense that support heavier gun restrictions, for instance.