Speaking of Heller, why do we need another ruling since we have Heller? I seriously don't know much about any of this stuff.
Heller addressed the issue of hand guns, and specifically a certain way in which they were "banned," (the locality basically not approving any permits for them).
In theory another locality could again pass a hand gun ban that would go into affect, and it would have to be challenged until a court found that Heller's decision controlled (or conversely, but unlikely, that Heller's decision did not control for some reason).
An AR ban was not discussed in Heller so the holding does not automatically extend.
However, based on the language in Heller, any court applying it as precedent and therefore it's logic, would be hard pressed to come to a different conclusion. Some have, though - and there are AR restrictions around the country that haven't been tested up to the Supreme Court yet (though no outright bans on ARs exist to my knowledge).
Heller says "Handguns cannot be banned because the Constitution protects commonly owned firearms used for lawful purposes. A handgun is a commonly owned firearm by and large used for lawful purposes. They are protected." Scalia deliberately crafted this language this way.
Well, anyone who tries to argue the AR-15 is different, but still acknowledges Heller as good law (which it is), would have to argue that if the Constitution protects commonly owned firearms for a lawful purpose, that the AR-15 is either not commonly owned or not used for lawful purposes generally.
But good luck arguing the AR-15 is not common... there are millions in circulation. And since handguns cause way more types of death and destruction, period, good luck arguing that they aren't used for lawful purposes. If handguns are mostly used for lawful purposes, so are AR-15s, there is no statistic that would rebuke that point.
His language is specifically designed to mean, yeah, like, maybe a flame thrower or grenade launcher is not commonly owned and used for lawful purposes, we can ban those.
You should take a half an hour and go read the opinion. It's not hard, and it's very good to be informed.