No question, "assault rifles" are now "firearms in common use"

Good deal. Yes, I have also seen AR15s in Walmart stores in Kentucky.

There is an IL Supreme Court case being briefed currently, that deals with this issue (Cook county's ban on certain semiautos). The defendants (the county) of course argue that such weapons are "dangerous and unusual" and thus not protected under the second amendment.

http://illinoiscarry.com/forum/index.php?app=core&module=attach§ion=attach&attach_id=7278

I use one every other weekend competing in shooting matches (USPSA Multi-gun). Several of the competitors are retired military, current police officers or sheriff deputies, and the rest are general civilians. If you live in a rural or semi-rural area near a major metropolitan area, it may take the sheriff 10 or 20 minutes to arrive. An AR15 or other suitable semi-auto rifle can be a good tool to defend your family from home invastion long enough to let the cops get there to help you. You just have to hold out long enough to let them get there. If you are in a very rural area, it can take even longer for the deputies or state patrol to get there with help.

These common weapons are called self-loading rifles, in widespread use for over 100 years. Ever since the Winchester Model 1905 Self-Loading Rifle, which was a centerfire rifle with a detachable box magazine.