Based on an earlier post, it looks like you're in Chicago like me. AR-15s are illegal here, as is the sale of spray paint...but I digress.
You are very limited in terms of the long arms (rifles and shotguns) you can own here. No handguns allowed, unless you're an LEO (law enforcement officer) or a City Alderman.
No magazine capacities over 12. No shotgun barrels under 18" or rifle barrels under 16". No pistol caliber carbines with the magazine well in the grip. In the semi-auto, could-potentially-be-considered-an-"assault weapon" category, you're pretty much left with a Mini-14...but no 20 or 30 round magazines. You can theoretically have an SKS, I believe, but it can't have a bayonet lug or flash suppressor or extended magazines. AR-15s are out.
Check out this site for more specifics--
Chicago ordinances. You really have to hunt for info. The City of Chicago website has nothing and the Gun Registration Office is criminally slow and not particularly helpful in helping identify what's legal and what isn't. A search for "gun registration" on the City's site will net you nothing, though registration is required. It's b.s. (balogna sandwich), I tell you.
Do get your FOID card, though. Takes about a month to get it back and is a veritable bargain compared to Chicago's gun registration fees.