Do I need a gun bag for my AR 15,or can I break it down and but it in my army bag.

This is more of an Illinois law question then a question about on target. do a quick google search for "transport of firearms in Illinois" and click the link for the Illinois DNR brochure, it should have the answer you are looking for.

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A gun case helps protect your weapon and optics so it doesn't bounce around. The trigger should locked and unloaded.