When it comes to legal language, armor-piercing bullets get all of the attention. This so-called "cop-killer" ammunition has been banned since 1986 because it can penetrate bulletproof vests. But in reality, many firearms used today have what gun enthusiasts call stopping power, a weapon's ability to render a target immediately harmless. To improve a gun's stopping power, you can do three things: increase the velocity of the bullet as it leaves the muzzle, increase the mass of the bullet being fired or design the bullet so it inflicts more trauma when it hits the flesh of the target. Rifles deliver more stopping power than pistols because they have much higher muzzle velocities. For example, the muzzle velocity of an M16 rifle is almost four times greater than a Colt M1911A1 pistol [source: Dougherty].
That doesn't mean pistols lack stopping power. In fact, most people who carry handguns for self-defense prefer larger-caliber ammunition -- .40 and .45 caliber and 9 and 10 mm -- to increase the odds that an assailant will go down and not get up [source: Kozak]. And what about gunmen bent on killing or wounding as many people as they can? They don't generally come loaded with .22-caliber target-shooting ammunition. Many of the U.S. mass shootings that have gained so much notoriety in the 21st century have included in the arsenal Glock semi-automatic pistols with larger, more powerful rounds:
- Adam Lanza, the shooter at Sandy Hook Elementary School in Connecticut, wielded a 10 mm Glock and a 9 mm Sig Sauer.
- James Holmes, the gunman accused of killing 12 moviegoers in Aurora, Colo., in July 2012, fired a .40-caliber Glock.
- Jared Loughner, the gunman charged with killing six and severely injuring others, including then-U.S. Rep. Gabrielle Giffords, at an Arizona shopping center in January 2011 used a 9 mm Glock.
- And Seung-Hui Cho, who gunned down 32 people in April 2007 on the Virginia Tech campus in Blacksburg, Va., carried a 9 mm Glock in addition to a Walther P22 pistol.
It should also be noted that Jared Loughner armed his Glock with a 30-round magazine. That configuration -- semi-automatic action combined with a high-capacity magazine -- would be considered illegal under the ban proposed by Sen. Feinstein. But the weapon's stopping power is also significant, which is why we include it as a hallmark of assault weapons.