So the primary reason millions of people own guns is to prevent the government from confiscating those same guns? That sounds a little circular to me.
People own guns for all kinds of reasons, from the eminently practical to the what's-he-compensating-for pathetic. My view is that the government must justify its actions to the citizens, and not the other way around. Americans shouldn't need to provide a reason for wanting a gun any more than they need to provide a reason for opting out of public school or wearing white after Labor Day. I don't think it matters that the founders didn't foresee automatic weapons any more than the First Amendment is invalid because they didn't foresee the internet.