Since most of the gangbangers are not buying their weapons through 'traditional' channels to avoid those time consuming Brady checks, what makes anyone think they are not buying their ammo from the same sources. Those that cause the most visits to the ER, would be paying NOTHING!
Why not put a .05 tax on crack and meth and let the users pay for the damage.
Or what about a tax on those do-rags the gangs wear on their heads?
A .10 levy on Malt beverages might be appropriate.
Why not a dollar tax on box cutters?