In his April ruling, however, U.S. District Judge William Young in Boston said popularity doesn’t matter.
[T]he AR-15’s present-day popularity is not constitutionally material,” he wrote in his ruling. “This is because the words of our Constitution are not mutable. They mean the same today as they did 227 years ago when the Second Amendment was adopted.”One of the recent SCOTUS decisions held that the 2A covered the keeping and bearing of any weapon "in common use" and was not limited to flintlocks. The AR pattern rifle being currently the most popular rifle in the country at the moment, it would seem to be covered.
OTOH, if as the judge says, "the words of our constitution are not mutable" then let's go back to the original document where it says "shall not be infringed." It seems like a good starting point.