Which, grammatically, isn't explanatory not operative. The operative part of the 2nd Amendment is, in full, "the right of the people to keep and bear arms shall not be infringed." In modern construction and words (based on Supreme Court decisions, specifically Heller and Miller) the full amendment would be something like, "Because a well-trained militia is necessary to the security of a free state, the right of individuals to keep and bear arms of military usefulness shall not be infringed." The first clause only explains why it's not be infringed; it doesn't impose a limit upon it.
Well, Miller did somewhat limit it based on the militia clause, by saying a firearm which wasn't demonstrated to be militarily useful was not protected, implying that if it had been demonstrated to be militarily useful it would be protected. So, under Miller, an assault rifle (obviously of military usefulness) would be protected but maybe not a break action shotgun. It's an odd case, at any rate, since Miller had died before it reached the Court and his side didn't argue before the Court.