Well....
There's a 10 day waiting period for all firearms sales in Washington State.
Rifles and shotguns only require the federal NICS check, if it's approved then you can take the gun home.
Washington State requires a separate background check for handguns.
As soon as the results of this check are known and you have passed NICS then you can take the handgun home or after a maximum of 10 business days have passed, whichever is lesser.
If you have a Washington State Concealed Weapons License then you have already passed the Washington State required background check and all you will need to do is pass the NICS and you can take the handgun home.
Screwy, but don't you dare question it.
https://app.leg.wa.gov/rcw/default.aspx?cite=9.41.092
RCW 9.41.092
Licensed dealer deliveries—Background checks.
Except as otherwise provided in this chapter, a licensed dealer may not deliver any firearm to a purchaser or transferee until the earlier of:
(1) The results of all required background checks are known and the purchaser or transferee is not prohibited from owning or possessing a firearm under federal or state law; or
(2) Ten business days have elapsed from the date the licensed dealer requested the background check. However, for sales and transfers of pistols if the purchaser or transferee does not have a valid permanent Washington driver's license or state identification card or has not been a resident of the state for the previous consecutive ninety days, then the time period in this subsection shall be extended from ten business days to sixty days.
[2015 c 1 § 4 (Initiative Measure No. 594, approved November 4, 2014).]
NOTES:
Finding—2015 c 1 (Initiative Measure No. 594): See note following RCW 9.41.010.