Many people equate assault weapon with machine gun -- a firearm capable of firing bullets as long as the operator squeezes the trigger or until the magazine empties. In reality, machine guns have been heavily regulated since the 1930s and pose less of a threat to civilians than semi-automatic weapons. A semi-automatic, or self-loading, gun is one that fires a single round every time the trigger is pulled. Confusion arises in the world of semi-automatic pistols, which also go by the name automatic pistols. In this case, however, what's automatic is how the gun loads, not how it fires.
According to the Congressional Research Service, U.S. civilians have access to 310 million firearms: 114 million handguns, 110 million rifles and 86 million shotguns [source: Krouse]. Although it's unclear how those numbers break down further, it's safe to say that semi-automatic pistols, rifles and shotguns are increasingly common today. This semi-automatic characteristic alone does not make a firearm particularly deadly, but when it's combined with high-capacity magazines (see No. 8) or certain aftermarket modifications, such as slide fire stocks (see No. 7), a mild-mannered gun can become an assault weapon.
Even without these extras, semi-automatic weapons deliver efficiency and rapid-fire results. Consider these sobering statistics: FBI studies have shown that an experienced gunman can fire a semi-automatic pistol six times in less than a second, which means he can empty a 20-round magazine in less than four seconds [source: Palmer].