Seems to me that anything that is used as a weapon, and performs like a weapon, is "weapons-grade anthrax" whether or not it is "run of the mill".

In theory perhaps, but it is my understanding that in order to be an effective weapon, it does have to be refined to the point where the particles are small enough to affect the lungs. As an example, a lump of plutonium could be used a weapon since it is radioactive, but in order to use it in a nuclear weapon it must be refined considerably. It is weapons-grade once it is suitable for use in a nuclear device.