Here's an excerpt~
*Some* AOW's are legal in IL. If you have questions, you might want to hit up a dude that uses the handle "Panzerfaust" on Subguns.com. He's an attorney that lives somewhere downstate.
The short of it is that as long as the AOW doesn't fall under IL's law of a short barreled shotgun (which is slightly different than the Fed. law and includes things the ATF would call AOW's) you are cool. So an AR15 with a front pistol grip, a semi MAC with a front pistol group, a smooth bore pistol, and stuff like that is kosher. Also, generally, DD's are legal in IL so you can have an M203 or a Boys .55 if you can find a sign-off or you go the corp. route.
Anyone that tells you different is ill-informed. As a side note, Phil brought up a good point about the registration numbers that ATF shows by state for different types of NFA. Me thinks that one of the things that got the suppressor exemption added for LE and dealers was the fact that ATF showed something like 400 of them in the state when it was technically illegal for ANYONE to have them.