Steve's correct. State law is adopted for National Parks. There's a lot of fine points to work out but it mostly only allows people with a California (!) county issued concealed carry permit to have a loaded weapon in a National Park -- though not in a building. Another federal law supersedes state law on that one (e.g. visitor center, admin center).
And, yes, people can do the open carry of an unloaded weapon, though I suspect they'd just subject themselves to weapons checks by officers all the time.
g.
None of the views expressed here in any way represent those of the unidentified agency that I work for or, often, reality. It's just me, fired up by coffee and powerful prose.
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