Residents will drive to Lake, Kane, Dupage, and Will counties for gun purchases (and pick up a carton of cigarettes on their way back home).
We experience the same thing in NJ with PA's state owned monopoly alcohol pricing model. PA booze buyers can be found at NJ liquor stores every day.
The down side is that owning a gun retailer in Cook County is becoming a failing proposition, it's best they relocate while they can.
It won't be the first business big government destroyed, and it won't be the last.