Reason.com, quoting 18 USC § 921, explains what constitutes a weapon of mass destruction (WMD) these days. The term “weapon of mass destruction” means destructive device.
The term “destructive device” means—
(A) any explosive, incendiary, or poison gas—
(iii) rocket having a propellant charge of more than four ounces,
(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,
(v) mine, or
(vi) device similar to any of the devices described in the preceding clauses;
So, anything that you can throw that contains more than a quarter ounce of explosive is a WMD. Eventually, the terrorism label will be so over-used as to be essentially meaningless. This is a common form of categorical inflation. If you have laws that criminalize everyone, the concept of crime becomes meaningless. If everyone is declared a hero, then the concept of heroism is meaningless.
Defining a quarter ounce of explosive as a WMD makes the concept of WMDs into a joke. I mean, even a bigger joke than what Bush made out of it when he used it to fabricate an excuse to start a war with Iraq.