You have to keep in mind that DD's are a federal classification. They include things such as, grenades, bombs, incindiaries, explosives, anti-personnel explosives, etc... They also include guns over .50cal that do not have a sporting-use (ie, .600 Nitro Express Elephant Stomper). [I'm heavily paraphrasing here folks]
The State of WA sees it differently. The state does not draw a distinction between a .49cal rifle and a .51cal rifle. To the State, a rifle is a rifle. On the other hand, most explosives are verboten with proper permits/licensing.
What does this mean? You can get a federal DD such as a 40mm Grenade Launcher (M203) or 60mm Mortar or 12Ga StreetSweeper and in the State of Washington you're good to go. But while federally you could ask for a DD Grenade, the State of WA would preclude it from happening unless you provided an exemption (ie, LE, Military, Demolitions, etc...)
In other words, DD's in the State of WA are limited to non-Explosives.