In recent discussions I've seen several people ask why anyone would need a firearm capable of firing more than N rounds (for arbitrary N) without reloading. The experience of Officer Peter Soulis provides some valuable lessons. While gunfights such as this are rare -- most defensive gun uses, by police or civilians, don't even result in shots being fired, much less the sort of firefight that happened here -- understanding worst-case possibilities is important for anyone who wants to be prepared for emergencies.
(The prose of this piece from Law Officer magazine shades to the purple -- the bad guy is grinning with blood lust, the hero full of calm resolve -- but let's not let that distract us.)
Soulis was shot during a traffic stop:
Instantly, he realized he'd made a grievous blunder. Grinning with blood lust, Palmer [a pseudonym -tms] lunged across the seat and shoved a Smith & Wesson Sigma up into firing position. Before Soulis could react, the S&W barked flame, driving a 9mm solidly into the center of his chest. The impact knocked Soulis back slightly, but his vest stopped the bullet.
Palmer was out of the Toyota a split-second later, firing the gun at him over the roof. There was no other cover nearby, so Soulis went down onto one knee behind the front fender to put the Toyota between them. But, at the same instant, two rounds crashed through his left arm, one just above the wrist and the other dead center on the forearm. Another struck him in the left thigh, although he wouldn't become aware of it until later.
...
Soulis was also becoming apprehensive about his wounds. The bullet hole in his left wrist was an ugly, swollen mess that made him wonder if he would have enough dexterity to reload...
As it turned out, Soulis was in fact able to reload. But the principle remains: a defensive shooter may be wounded and unable to reload. That's something to take into account.
Soulis returned fire:
Soulis was shooting back now, pumping rounds through the windshield into his assailant. Palmer went down immediately, and Soulis used the opportunity to seek better cover....
...
Soulis knew he'd have trouble tracking Palmer if he came around that way, so he decided to make his move without delay. He lunged out from behind the car, thrust the Glock up into firing position, and opened fire. His first two rounds hit Palmer center chest, rocking him back on his heels. Palmer flinched as two more rounds hit center mass, and then started backpedaling toward the Toyota. He was still holding his gun, but never raised it to fire.
...
Soulis stopped and fired two rounds through the back window. The first missed, but the second hit Palmer in the upper back, driving his head forward into the steering wheel. That seemed to have done the trick, but then Palmer sat up again, dropped the transmission into reverse, and started backing up. With no time to ponder how Palmer had absorbed so many hits, Soulis took aim and emptied the magazine into his assailant.
Palmer rolled over to his right and dropped the gear shift lever into drive, causing the car to lunge forward into a chainlink fence a few feet away, where it came to a stop.
In all "Palmer" (not the attacker's real name) was hit by 22 .40-caliber rounds, 17 of which were "center mass" hits near the heart, lungs, and spine. After the last of those hits he was still capable enough to shift the car. An autopsy showed that he was not under the influence of any drug other than a small amount of alcohol.
Again, this is a highly exceptional case, a statistical outlier. Usually if shots must be fired one or two is sufficient. For example, an analysis of five years of reports in the NRA "Armed Citizen" column found that when shots were fired, the average number was two. Caveats: there is a selection bias here, and the period analyzed was 1997-2001. Many states have liberalized CCW laws since then, which may have changed the circumstances of encounters.
And for police, an analysis of data from the NYPD shows that for 2006, the average number of shots fired by officers was 4.7. That was skewed by the infamous shooting of Sean Bell; excluding that one incident where fifty shots were fired, the average would be 3.6.
So this case is enough of an outlier that, if I carried a gun (CCW permits are just about impossible for the average civilian to get in Maryland) I wouldn't feel ill-prepared with a lower capacity firearm.
But Officer Soulis's experience shows that sometimes, you need to be able to shoot a whole bunch of rounds very quickly, and I won't condemn anyone who wants to be prepared for unlikely circumstances. That is, after all, the whole point of emergency preparedness.
Meanwhile, for a mass shooter confronting unarmed victims, a "high capacity" magazine doesn't seem to add to their lethality. Such shootings are rare enough that any conclusions have to be tentative (and that gun control laws based on stopping them can by nature have little over-all impact). But while a few shooters have been stopped while reloading, in many cases mass shooters have been able to reload several times (even using revolvers and speed-loaders), while in other cases shooters with rounds still left in their weapons have been stopped by unarmed civilians.
Such magazines would be easy for black-market gunsmiths to make (they can even be made on a 3D printer now) -- but they exist in such vast quantities already that that's unlikely to ever be necessary to supply those who want them. Such bans take money and manpower to enforce that could be better spent elsewhere, and result in people of no ill will who violate technical restrictions getting arrested -- unless they happen to be, say, influential members of the press.