How did they do that? Because of the overly broad wording of their new assault weapons ban (edited to clean up the text to make it more readable, but not change the wording):
(3) "Assault weapon" means:
(snip of irrelevant sections A and B)
(C) A semiautomatic rifle that can accept or can be modified to accept a detachable magazine and has at least one of the following:emphasis added
(i) A folding, telescoping, or collapsible stock.
(ii) Any grip of the weapon, including a pistol grip, a thumbhole stock, or any other stock, the use of
which would allow an individual to grip the weapon, resulting in any finger on the trigger hand in
addition to the trigger finger being directly below any portion of the action of the weapon when firing.
Now picture any semiauto rifle you can think of. Now tell me which one, if any, sees the pinkie finger of the trigger hand not being below the action of the rifle.
Thus, all semiauto rifles in Illinois are now legally defined as assault weapons and are thus illegal to possess, transfer, or own.