The text of HB5855 has a 300 day grace period for so-called Assault Weapons and .50 cal -- but does NOT have any grace period for magazines capable of holding more than 10 rounds. Possession of two or more mags capable of holding more than 10 rounds is a Class 2 felony. There are millions of people in Illinois who would be instantly felons as a result.

from the bottom of the link above:

(b) Except as provided in subsection (c), it is unlawful for any person within this State to knowingly manufacture, deliver, sell, purchase, or possess or cause to be manufactured, delivered, sold, or purchased a large capacity ammunition feeding device.

(d) Sentence. A person who knowingly delivers, sells, purchases, or causes to be delivered, sold, or purchased in violation of this Section a large capacity ammunition feeding device capable of holding more than 10 rounds of ammunition commits a Class 3 felony for a first violation and a Class 2 felony for a second or subsequent violation or for delivery or possession of 2 or more of these devices at the same time