This is a scary case. Given the lean of the supreme court recently, this could end very, very badly for gun owners. If the supreme court decides that they are protected, awesome. If they decide they aren't, get ready for a witch hunt of epic proportions.
If the central argument in this case is that AR15's fall into the common use category and therefore shouldn't be prohibited, I don't see any way the court could decide that they aren't in common use. I can't find figures that state how many AR15's are sold in the US each year, but it easily numbers in the millions.
This is a gun forum, so the numbers will be a bit slanted, but how many people do you know that own at least one AR15? I can think of a dozen folks off the top of my head. Most of those own a few of them. My 79 year old step-dad, whose primary interest in firearms is collecting early semi-automatic pistols, keeps three different AR15's stashed around the house to protect he and my mother.