While Illinois may have some wacky laws, simply passing through with properly secured firearms is not illegal. From the Illinois State Police website (
http://www.isp.state.il.us/foid/firearmsfaq.cfm):
How can I legally transport a firearm on my person or in my vehicle?
There is more than one way to legally transport a firearm. However, in order to be in compliance with all statutes, it is recommended all firearms be transported:
Unloaded,
Enclosed in a case and,
By persons who have a valid FOID card.
What constitutes a legal "case" for transporting a firearm?
The Criminal Code refers to "a case, firearm carrying box, shipping box, or other container." However, the Wildlife Code is more specific, defining case as "a container specifically designed for the purpose of housing a gun or bow and arrow device which completely encloses such gun or bow and arrow device by being zipped, snapped, buckled, tied, or otherwise fastened with no portion of the gun or bow and arrow device exposed."
If a non-resident is coming to Illinois to hunt and would like to bring their firearm, how do they legally transport it?
Non- residents must be legally eligible to possess or acquire firearms and ammunition in their state of residence. It is recommended that, in order to be in compliance with all statutes, non-residents transport all firearms:
Unloaded, and
Enclosed in a case, and
Not immediately accessible or broken down in a nonfunctioning state.
Is it legal to have ammunition in the case with the firearm?
Yes, so long as the firearm is unloaded and properly enclosed in a case.
Your trip may not take you through Illinois, so this may all be a moot point. But if you decide to pass through the Land of Lincoln, you should be good to go (save for that Glock in your glovebox...might wanna put it in the trunk).