You made the assumption that I want to get rid of all the guns. I have no such desire. My intention is to make sure that guns stop causing harm that they do not pay for.

Your brother-in-law may be correct, but I don't think anyone has collected the kind of data required to make that kind of assertion. Also, we would need a fairly detailed definition of what "responsible gun carriers" are, and have some way of verifying whether somebody was continually meeting those requirements.

You've made another assumption, which is that anything that is a public good must be subsidized. Let's assume responsible gun ownership is a public good. So what? That does not imply that there would be a tax on everyone who doesn't buy a gun. Most public goods are not subsidized to social optimum levels. This would likely be no different.

Rather, gun owners would effectively be subsidizing the state by taking care of a part of the crime problem with their responsible gun ownership. They already seem perfectly happy to do so, so it's a win for everyone.