The man who fatally shot nearly two dozen people at an El Paso Walmart on Saturday, alleged to be a right-wing extremist, and the man who shot and killed nine people only hours later in a downtown area of Dayton, Ohio, both unleashed their savage attacks thanks to military-style rifle s. Just as culpable for the carnage, however, were the large-capacity magazines that enabled these shooters to discharge many rounds of ammunition without reloading.

Once all the rounds in a magazine are fired, reloading takes time. That chunk of time is often the crucial moment in which citizens are able to flee (or fight) and law enforcement is able to arrive and gain some control of the situation. When shooters can fire off dozens of shots before reloading, the potential for mass casualties heightens.

The lethality of military-style rifles is self-evident: Six of the deadliest mass shootings in the last 10 years all included military-style firearms. But large-capacity magazines — generally defined as ammunition-feeding devices holding more than 10 rounds — are arguably even more dangerous than the guns themselves: A study last year found roughly half of recent mass shootings involved them. A growing consensus among criminologists is that, as deadly as military-style weapons are, the critical factor that multiplies the mayhem is not necessarily the style of the weapon but the size of the magazines.

The tragic use of large-capacity magazines isn’t limited to mass shooters, who for all their destruction are still responsible for only about 1 percent of all gun killings in a given year. The criminologist Christopher S. Koper — who researched the impact of the 1990s federal assault weapons ban, a law which also limited magazines to 10 rounds — published a paper last summer showing that since the federal ban lapsed in 2004, gun crimes committed with large-capacity magazines have increased steadily; over 40 percent in cases of serious violence. Mr. Koper and his co-authors also reported, “Trend analyses also indicate that high-capacity semiautomatics have grown from 33 to 112 percent as a share of crime guns since the expiration of the federal ban — a trend that has coincided with recent growth in shootings nationwide.” A 2014 study concluded that “ large capacity magazines were used in more than half of all cases with significant increases in fatalities, injuries, and total victim counts identified.”