Stephen quipped in his essay that if you can't kill an intruder (or your wife in the act of raiding the fridge) with 10 rounds, you need to go back to the shooting range.
In response to that assertion, I'd like to point out the recent incident in Georgia where a woman successfully defended herself and her two nine year old children from a crowbar wielding intruder. She fired all six rounds from her .38 revolver, striking the man in the face and neck with five rounds, after retreating into a crawl space with her kids to avoid him. Yet, the intruder was able to flee the home and drive off. Five .38 caliber rounds did not instantly incapacitate him. Luckily, he did not choose to continue to press the attack, instead opting to flee.
What if there had been more than one intruder? What if she had hit with fewer of her six rounds, or perhaps missed with all of them? A Glock 19 with 15 rounds in the magazine, and one in the chamber, would have given her 10 more rounds, and much better capability of defending herself and her children. Also, a couple of spare magazines kept handy would provide far quicker reloads, than even speedloaders can offer with revolvers.
Many people do not have the time, or can afford, to frequent a shooting range to keep their pistol marksmanship sharp. High capacity magazines give a less skilled shooter a greater chance of disabling an intruder. I agree that anyone who assumes the awesome responsibility of keeping a loaded firearm in their homes, or legally carried concealed on their person, should train enough with the weapon to be as proficient as possible. But, even hundreds of hours of shooting at static paper targets cannot ensure deadly accuracy with a handgun, when the targets are violently animated, charging at you with a knife, or firing back at you. When confronted with a life threatening situation, the adrenaline flows preparing us for for "fight or flight". Heart rates soar, and blood flow is restricted from the extremeties to mitigate blood loss when injured. There is a loss of fine motor control, which makes hitting anything with a pistol even more difficult. All of which makes choosing a pump action 12 gauge shotgun, or an AR-15 with 30 round magazines a better choice for home defense, than a revolver. A long gun is much more accurate and controllable, because it has a much longer sight radius (the distance between the front and rear sight), a longer barrel that stabilizes the projectile/s for a longer time, and is steadied against the shoulder. It also justifies high capacity magazines.
One could maximize one's firepower, within the proposed 10 round limitiation, by choosing the largest caliber handguns available, such as .45 ACP or 10mm, as opposed to 9mm or .40 S&W. However, the larger the caliber, the greater the recoil, and thus increased challenge to accurate shot placement.
The medical examiner on the scene at Sandy Hook reported that the bodies he examined had been shot with the Bushmaster 5.56mm rifle 3 to 11 times. The most widely available magazine for that rifle hold 30 rounds (though there are some higher capacity magazines and drums that hold 40, 60, or even 100 rounds). So, even with the 30 round capacity, Adam Lanza swapped out magazines several times to kill 26 people. Would the carnage have been lessened if he only had 10 round magazines? Would he have been mauled by an organized pile on of rallying 6 year olds, if he'd had to swap magazines more frequently? Or, would his mother have opted for purchasing a higher caliber rifle, like 7.62mm or even larger, if the advantage of higher capacity wasn't legally available? If so called "assault" rifles were banned altogether (like they were in the state of CO at the time), and Adam Lanza had instead carried the 12 gauge shotgun into the school that day (that was in the trunk of his car), would the death toll have been any less? 12 gauge 00 Buck contains nine .33 caliber pellets in a single shell, which is capable of killing a man the size of Hulk Hogan, and any six year old. The magazine tube on my Mossberg "Persuader" 12 gauge can hold 7 shells, plus one chambered. One can outfit the shotgun with shell carriers on the receiver, and a sling with loops that hold shells, and wear bandoleers over your shoulder that hold at least 50 shells. It's not that difficult to slip shells into the shotgun, loading as you're firing.
Competition pistol shooters can slap fresh magazines into their semi-auto pistols in the blink of an eye. Limiting magazines to 10 rounds won't make a dent in the death tolls of these rampage shooters. They'll just bring more magazines, and more pistols, and probably opt for higher calibers. A single well placed .45 ACP is plenty lethal.