From the IL DNR 2019 Deer Hunting Fact Sheet.....
LEGAL HUNTING DEVICES • Shotguns, loaded with slugs only, of not larger than 10 nor smaller than 20 gauge, not capable of firing more than 3 consecutive slugs; or • Single- or double-barreled muzzleloading rifles of at least .45 caliber shooting a single projectile through a barrel of at least 16 inches in length; or • Centerfire revolvers or centerfire single-shot handguns of .30 caliber or larger with a minimum barrel length of 4 inches; or • On private land only, archery equipment may be used with a FIREARM permit ONLY during the Firearm Seasons Nov. 22-24 & Dec. 5-8.
From the 2019/2020 IL Hunting and Trapping Digest....
Can I use a centerfire rifle for hunting in Illinois? Or handguns? A: The following species may be harvested with a rifle or handgun: coyote, striped skunk, woodchuck, squirrel, rabbit, raccoon, opossum and red and gray fox. We do not regulate the caliber of handgun or rifle that may be used, except on stateowned or managed areas, which normally allow only the use of shotguns or .22 caliber rifles. Squirrel, furbearers and woodchuck hunters may use a .17 caliber rifle on some state-owned or managed areas. No rifles may be possessed in the field, however, during firearm deer season except trappers can use guns with a caliber no larger than .22 during deer season to dispatch trapped furbearing mammals. Certain handguns are lawful for deer hunting. At all times when in possession of firearms, hunters must comply with all firearms regulations as outlined in the Illinois Criminal Code
From the IL DNR Website.... << SAD >>
Illinois Digest of Hunting and Trapping Regulations (Spanish)