A semi-automatic M16 assault rifle with a scope and extra magazine. (Above right) 5.56mm cartridges.
"Gun Show loophole"
Current federal law requires prospective gun buyers undergo a criminal background check before purchasing a firearm from a licensed firearm dealer. But if the buyer wanted to buy a gun through any private channel — a newspaper ad, a website, or most commonly an unlicensed seller at a gun show—there would be no such check in most states. An estimated 40 percent of gun sales go through these unlicensed channels, according to government statistics—and the loophole exists in 33 states. Bills proposing to close the loophole by requiring background checks for all gun purchases have been introduced in the House and Senate, though neither has made it out of their respective committees.
This refers to the diameter, in inches or millimeters, of a weapon's barrel or the cartridges it fires. Lanza's Bushmaster rifle used .223-caliber ammunition, meaning its rounds were .223 inches wide. Typically this spoken as a "two twenty-three rifle."
Lanza also used two handguns: a Sig Sauer 9mm and a Glock 10mm. These round sizes are measured in millimeters as opposed to inches. Sometimes, handguns are referenced in caliber instead of the metric millimeter. The .357 ("three fifty-seven") .38 ("thirty-eight") for example, are popular handgun and cartridge sizes.
The caliber of a firearms does not indicate its exact appearance, which is why attempts such as this, which aim to guess Lanza's weapon based solely on its caliber, were misguided.
Unlike with rifles and handguns, shotguns' calibers are classed by "gauge," which refers to the diameter of the barrel rather than the size of the round it fires.
Seth Cline is a reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.