That's your interpretation Harvey, surely? The law says a missile, singular, like a bullet.
Dust shot individually are pretty harmless at range. Obviously if you fire a single, solid mass you may put it over the limit and render it illegal - just like owning a functioning air cane is legal until you actually use it.
Don't forget, the law tolerates all kinds of devices that deliver missiles of high KE but generally low accuracy, like a catapult, slingshot, or even a throwing hammer! The reason guns are singled out for restrictions is that they can potentially deliver destructive missiles with accuracy at range.