Also constitutionally suspect are restrictions on magazine size. There is no question that a limit of 10 rounds (as the president has proposed) or seven (as enacted by New York state last month) would impair the right to self-defense. A magazine with 10 rounds may provide adequate protection against a single nighttime intruder. But it may not: What if there are two intruders?

Further compounding the constitutional problem is the fact that the benefit of such limits is questionable. For a practiced and calm shooter, swapping magazines takes no more than a couple of seconds. And a swap may not even be necessary if the shooter has multiple guns, as in several mass shootings in recent years.

While some limit on magazines may be constitutionally permissible, one that falls below the capabilities of guns in common usage for self-defense is probably not. The most popular guns for self-defense take 15 or so rounds in their default configurations. Given the uncertain benefit of restricting magazine size, not to mention the tens of millions of “high capacity” magazines in circulation, something near that number may be a constitutional minimum.

And while there is no question that procedural requirements like background checks are permissible, that does not mean that the government may place an undue burden on the right of law-abiding citizens to protect themselves.