How can a free society protect itself?

Maybe it will, maybe it won’t.

The primary cost of maintaining coercive controls on anything is liberty. I won’t give up liberty for any trade-off.

Ah, and here’s the crux of the problem. Let’s use your line. Ok, got it, line in place - detonators can be controlled because, well, I’m not sure why but that’s what you said. Firearms cannot be controlled because, well, I like freedom. Ok. Now make it work. How do you plan to police or enforce your line? What do you do when the government steps over it? By definition, they’re better armed than you are, because they just banned the use of some powerful weapons by others.