Yes, exactly, you got it exactly right: whether the founding documents were ever useful, what governing principles do we want to live with now?
We obviously constrain people's agency in lots of ways that are non-controversial, like requiring that people drive on one side of the street or the other, or requiring that people not dump toxic waste in public parks. How do guns fit into that picture? How does speech, for that matter?
These are good and important questions to think deeply about, and I'm so glad you raised the issue so articulately.
My background, by the way: not a gun owner now, but I've used them enough to have a pretty good familiarity with them (and was once licensed to do armed security work, even). I'd fully endorse gun ownership restrictions, background checks, etc. I have never seen any rational argument against such laws; to me, it just goes to show how the NRA has done an amazing job conning a lot of people into becoming their own private gun lobby army, heh heh.