From Whidbey News Times:

The definition of firearm contained in state law and in I-594 reads, “… A weapon or device from which a projectile or projectiles may be fired by an explosive such as gunpowder.”

 These flare guns have previously been determined to be firearms by the WSP Crime Lab in Tacoma because they fire a projectile by an explosive.

Home Depot, Lowe’s and other hardware stores sell Ramset nail guns, which use a gunpowder charge to fire nails, usually into concrete or steel.