It's still considered a firearm in IL. The definition of a firearm in IL is: 430 ILCS65/1.1
"Firearm" means any device, by whatever name known, which is designed to expel a projectile or projectiles by the action of an explosion, expansion of gas or excape of gas; excluding, however:
(1) any pneumatic gun, spring gun, paint ball gun or B-B gun which either expels a single globular projectile not exceeding .18 inch in diameter and which has a maximum muzzle velocity of less than 700 feet per second or breakable paint balls containing washable marking colors;
Therefore, even a .22 caliber pellet gun is considered a firearm.