The tax idea is not a deterrent. The real criminals won't buy from a legitimate dealer. So the guns you're going after, will only be known of when they're used, and only if they catch them.
going after guns is not going after criminals. It's a waste of time and money.
The only solution that will be effective is something that can actually weed out the loonies and criminals. It won't happen by punishing everyone or a select few. And again, the people you intend to tax, in all likelyhood won't ever pay it. The only ones that will pay is the already law abiding. It's pointless to go after the law abiding and is a component of bad law.