I think this is what you are looking for to justify protecting yourself when you feel there is an actual threat but later find out it was an airsoft gun.
RCW 9A.16.050
Homicide — By other person — When justifiable.
Homicide is also justifiable when committed either:
(1) In the lawful defense of the slayer, or his or her husband, wife, parent, child, brother, or sister, or of any other person in his presence or company, when there is reasonable ground to apprehend a design on the part of the person slain to commit a felony or to do some great personal injury to the slayer or to any such person, and there is imminent danger of such design being accomplished; or
(2) In the actual resistance of an attempt to commit a felony upon the slayer, in his presence, or upon or in a dwelling, or other place of abode, in which he is.
Washington Practice Series TM
Current through the 2010 Pocket Parts
Washington Pattern Jury Instructions--Criminal
2008 Edition Prepared by the Washington Supreme Court Committee On Jury Instructions, Hon. Sharon S. Armstrong, Co-Chair, Hon. William L. Downing, Co-Chair
Part IV. Defenses
WPIC CHAPTER 16. Justifiable Homicide
WPIC 16.07 Justifiable Homicide—Actual Danger Not Necessary
A person is entitled to act on appearances in defending [himself][herself][another], if that person believes in good faith and on reasonable grounds that [he][she][another] is in actual danger of great personal injury, although it afterwards might develop that the person was mistaken as to the extent of the danger.
Actual danger is not necessary for a homicide to be justifiable.
NOTE ON USE
Use this instruction with WPIC 16.02, Justifiable Homicide—Defense of Self and Others, and WPIC 16.03, Resistance to Felony, when appropriate.