SPRINGFIELD — The Illinois Supreme Court ruled Thursday that a Cook County tax on gun purchases is unconstitutional, but it left the door open for a more tailored tax that specifically goes toward mitigating gun violence and its effects.The Cook County gun tax, which took effect in April 2013, imposed a $25 fee for retail gun purchases in the county, as well as a 5-cent fee per cartridge of centerfire ammunition and 1-cent-per-cartridge fee for rimfire ammunition.The taxes were challenged by the trade group Guns Save Life …