Capacity is important.

Would you want your car’s gas tank to only hold 1 gallon of gas? No, you prefer it to hold 12-20 gallons; enough to get you around 300 miles, not 30 — most people couldn’t do what they need to do in a day if the car only held enough gas to do 30 miles at a time. It’s also one barrier with more widespread adoption of electric cars, because you just can’t go very far. This is why hybrid cars exist, to help overcome the capacity issue.

Even if you don’t like the notion of humongous drink cups (e.g. 44 oz, 64 oz, etc.), you still prefer some sort of capacity when you want a drink, especially if you need to take that drink with you. I mean, if you could only get your coffee in thimble-sized shots, there’d have to be a Starbucks every 50 feet, amiright? You prefer your Grandes, your Ventis, and perhaps your Trentas, don’t you? Makes life better.

Have you ever noticed that police don’t carry single-shot guns? Why is that?

Whether a gun is used as a deterrent (simply by being present) or is used in the most common fashion a confrontation (drawn but not fired), or is used by pulling the trigger, the objective is always to stop the threat. In situations where the gun must be fired, not all shots hit an attacker. Of those that do, few are immediately incapacitating, unless they strike the heart or the central nervous system. Yet when the defender has sufficient reserve capacity, even multiple attackers may decide to desist from an attack. That is why American law enforcement officers ubiquitously carry handguns with more than ten rounds of ammunition, and often more than 15. It is also why their rifles typically have 20 or 30 round magazines, not 10.

If the firing of several shots has wounded one attacker, and has resulted in the other attacker putting up his hands, the defender needs to control the situation until the police arrive. That is why reserve capacity is so important for law enforcement and for citizens. Reloading is very difficult when the second hand is holding a cell phone. Even a two-handed reload will likely make the gun temporarily inoperable and cause the gun to move off target for at least a few seconds, giving the criminal(s) a new window of opportunity. Citizens do not carry police radios, and police response to a cell phone call about citizen in trouble is often slower than the response to a radio call about an officer in trouble. The reasons why magazines for greater than 10 rounds are the overwhelming choice for law enforcement officers for lawful defense of self and others apply a fortiori to citizens, who rightly  look to law enforcement officers as good models for gun safety practices.

– David Kopel, “The history of magazines hold 11 or more rounds: Amicus brief in 9th Circuit”

Mr. Kopel wrote this brief (and this article) because there’s a court case where a ban on magazines due to their capacity was being upheld in part because the Court’s analysis stated that magazines did not exist at the time the Second Amendment was ratified. Mr. Kopel gave a history lesson, showing how such technology existed as early as the 16th century – a couple hundred years before the USA Founding Documents were authored. It’s an interesting read on history.