The common use test addresses present and (presumably) past arms, but what about the future? The court is clear that the 2nd Amendment will protect technology yet to be invented. For that purpose, the common use test is wholly useless. At best, the common use test is but one measure of a weapon's 2A protection.
AR15s, (and I would argue any weapon which shares its performance characteristics, regardless of the technology through which it operates) clear the common use hurdle by a mile. A weapon cannot simultaneously ubiquitous AND unusual.
But as new technology is introduced, there will be the inevitable period of time where it is initially rare in the population at large. It will be interesting to see what tests are concocted for that eventuality.