|Is this something that is common use for lawful purposes?|
It looks like the issue of states enacting "assault weapons bans" just might be heading to the Supreme Court. When DC v. Heller was decided back in 2008, the Supreme Court held that the Second Amendment was a Constitutional right unconnected with a requirement that a citizen serve in the military. Basically, the Court confirmed that individual citizens are protected by the Second Amendment, which to me, seems obvious.
In any event, since the Supreme Court has ruled that the Second Amendment applies to everyone, the new fight is shaping up over what sort of firearms may be banned by states.
Back in December, the Court of Appeals for the Seventh Circuit held that a Chicago suburb could ban "assault weapons" which had been essentially defined as AR-15s and things that looked like AR-15s. The Seventh Circuit Court of Appeals is the appellate court for federal cases in Indiana, Illinois, and Wisconsin.
So here, we had one federal appellate court holding that the Second Amendment does not prohibit a ban on "assault weapons".
Just last week, an equal court - the Court of Appeals for the Fourth Circuit - held that Maryland's almost identical 2013 law prohibiting "assault weapons" violated the Second Amendment, finding that the prohibited firearms are in common use for lawful purposes, and are therefore protected by the Second Amendment. The Fourth Circuit Court of Appeals is the appellate court for federal cases in Maryland, West Virginia, Virginia, North Carolina, and South Carolina.
As you can see, we now have two equal courts in different areas of the country coming to opposing conclusions of law on the same topic. This is what is known as a "circuit split". For obvious reasons, the Supreme Court doesn't want federal law to be different in different areas of the country. A circuit split is one of the regular reasons that the Supreme Court will hear a particular case. It allows the Supreme Court to take the issue and set forth the single standard that will apply in all circuits.
And on something as hot-button as gun control, my guess is that the Supreme Court will likely take this case to do exactly that.