The Second Amendment protects "the right of the people to keep and bear Arms." But it has little to say about the ammunition for those arms. Given that just about every gun that existed at the time the Constitution was written fired a single shot at a time, it's likely that the framers didn't envision large capacity magazines feeding dozens or hundreds of rounds of ammunition into a single firearm, much less protecting the right to those magazines.
But that's not quite why the U.S. Third Circuit Court of Appeals upheld New Jersey's ban on magazines housing more than 10 rounds of ammunition. Instead, the court cited the recent rise in active and mass shooting incidents, and the common theme that many of those mass shooters take advantage of large capacity magazines (or LCMs), leading to a "significant increase in the frequency and lethality of these incidents."
More Shots = More Casualties = More Regulation
"LCMs allow for more shots to be fired from a single weapon and thus more casualties to occur when they are used," wrote U.S. Circuit Judge Patty Shwartz. "By prohibiting LCMs, the Act reduces the number of shots that can be fired from one gun, making numerous injuries less likely." Also important to the Third Circuit was the rapid rise in the frequency and lethality of mass shooting incidents:
Active shooting and mass shooting incidents have dramatically increased during recent years. Statistics from 2006 to 2015 reveal a 160 percent increase in mass shootings over the prior decade. Department of Justice and Federal Bureau of Investigation ("FBI") studies of active shooter incidents (where an individual is actively engaged in killing or attempting to kill people with a firearm in a confined, populated area) reveal an increase from an average of 6.4 incidents in 2000 to 16.4 incidents in 2013. These numbers have continued to climb, and in 2017, there were thirty incidents. In addition to becoming more frequent, these shootings have also become more lethal ... (citing 2018 article noting "it's the first time [in American history] we have ever experienced four gun massacres resulting in double-digit fatalities within a 12- month period").
That was enough to justify New Jersey's interest in limiting the capacity of ammunition magazines. "New Jersey's law," the court concluded, "reasonably fits the State's interest in public safety and does not unconstitutionally burden the Second Amendment's right to self-defense in the home."