Re: "What do you tell people..."
My response depends on who's asking, some have asked for very obvious and mean spirited reasons.
For these, I have borrowed from the Jeepers favorite line (I like it anyway)
"It's a gun thing, you wouldn't understand"
Most of my debates with those who don't 'get it' has convinced me the average anti-gunny is poorly educated in our Constitution, though they may have been well educated in other things.