There are some serious downsides. In this case, this should get progressive alarms going off.
But before we get to the bulk, I'm going to repeat my last line's question first. Why invent new ways to fuck the poor in the name of gun control when we have solutions that work?
It encourages transitioning gun ownership percentage to wealthy white people and less gun ownership to less-wealthy and non-white people (who, on average, make less money). This is the big one
People don't like to admit it, but gun ownership DOES have a deterrent effect in high crime areas. Home invaders regularly mention avoiding houses of armed people when interrogated. I don't want ANYONE robbed on my street, but I definitely don't want my family victimized. My road has a dramatically lower home invader rate (based on value of property) than surrounding areas. Why? Outspoken gun owners due to the hunting culture (we have too many deer)... Do we really want to pretend to justify all the upsides of gun ownership to going to rich white people?
Over 90% of gun crimes are committed with illegal weapons, a majority of which go back to legitimate owners and were stolen/given illegally. That means the liability insurance chain is already broken (or the rates go up, further alienating poor folks)
Simply tracing, background checks, and better regulation all-round would be more effective than a regressive tax on gun ownership. And those things are well-established and well-tested in society. Regulations WORK. So why invent new ways to fuck the poor in the name of gun control when we have solutions that work?
EDIT:
And some other thoughts that kinda go both ways at once. It looks like $300k is the quoted amount by most 2A firearm insurance companies. Almost like they lobbied for the bill. It makes me wonder if they would also lobby for weakening other regulations because "well gun owners are insured".
And part 2 as a flipside. It looks like the costs might not be terribly high. I'm seeing quotes as low as $30/mo. It's hard because they are all EXTREMELY shadey companies and (like other insurance companies) they like to hide their rates from potential buyers. As well as their fine print since the rates are so low from them avoiding paying out. By their fine print, it looks like they don't pay out if your action might have been criminal. So the insurance doesn't actually pay the victims of anything except accidental discharge.
But then, do we want to empower another questionably corrupt industry by mandating gun owners be their customers?