Why not a got-caught-committing-a-crime tax?
Shocking, I know.
While it is a small amount on a $500 purchase, I'm sure a lot of gun shop business will go to out of county shops on the principle.
How is the economic health of Chicago these days? Is it declining or robust? Seems they try hard to drive away sane people, so I would imagine it has got to affect their tax base. Not sure if this is really primarily a gun control measure or "we are so desperate for tax revenue to pay off our mushroom cloud of debt, let's find another tax to levy!"