The law defining 'weapons of mass destruction' was written in 1994, but does in fact reference pretty much any 'destructive device.' That latter term was first defined in 1968, and includes things like hand grenades or things with as little as a quarter-ounce of explosive material.
The 'mass' part reflects the principle of damage radiating outward from the origin and affecting whoever/whatever happens to be in proximity, as opposed to the more targeted destruction caused by a bullet or handheld weapon. Of course you could kill multiple people with a handheld weapon or (rarely) hit several people with a single bullet, but in general such attacks involve one-on-one interactions even if there is a series of them in rapid order, as with automatic gunfire. hitting multiple individuals from a single shot or manual attack is considered a fluke.
Note that the law is not necessarily drafted to reflect the actual probability of injury, but to dissuade any use of the method; explosive, chemical, biological and radioactive attacks are by nature indiscriminate. That's considered much worse than an attack on a particular individual (you could launch an indiscriminate attack by, say, firing a gun into a crowd. But that lack of care about who you hit would itself attract a higher sentence). Furthermore, it radiates temporally; if you plant a device and then drop dead of a heart attack, the device remains dangerous even though you are no longer capable of forming criminal intent. It presents risk from when it's emplaced to when it has finished releasing, a period which could last years. In similar fashion, using a bomb or poison to kill someone automatically makes the offense into Murder in the First Degree, negating defenses like 'crime of passion' or 'heat of the moment'.
Comments
The law defining 'weapons of mass destruction' was written in 1994, but does in fact reference pretty much any 'destructive device.' That latter term was first defined in 1968, and includes things like hand grenades or things with as little as a quarter-ounce of explosive material.
The 'mass' part reflects the principle of damage radiating outward from the origin and affecting whoever/whatever happens to be in proximity, as opposed to the more targeted destruction caused by a bullet or handheld weapon. Of course you could kill multiple people with a handheld weapon or (rarely) hit several people with a single bullet, but in general such attacks involve one-on-one interactions even if there is a series of them in rapid order, as with automatic gunfire. hitting multiple individuals from a single shot or manual attack is considered a fluke.
See notes on http://www.law.cornell.edu/uscode/text/18/2332a and http://www.law.cornell.edu/uscode/text/18/921
Note that the law is not necessarily drafted to reflect the actual probability of injury, but to dissuade any use of the method; explosive, chemical, biological and radioactive attacks are by nature indiscriminate. That's considered much worse than an attack on a particular individual (you could launch an indiscriminate attack by, say, firing a gun into a crowd. But that lack of care about who you hit would itself attract a higher sentence). Furthermore, it radiates temporally; if you plant a device and then drop dead of a heart attack, the device remains dangerous even though you are no longer capable of forming criminal intent. It presents risk from when it's emplaced to when it has finished releasing, a period which could last years. In similar fashion, using a bomb or poison to kill someone automatically makes the offense into Murder in the First Degree, negating defenses like 'crime of passion' or 'heat of the moment'.