RCW 9.41.092: (2) Except as otherwise provided in this chapter, a licensed dealer may not deliver a semiautomatic assault rifle to a purchaser or transferee until ten business days have elapsed from the date of the purchase application or, in the case of a transfer, ten business days have elapsed from the date a background check is initiated.                                                                                                        
      It has been 6 business days since I initiated my background check with a home FFL that I've gone through once before. He called me this morning to say that my rifle is ready for pick up because my Background check came back clean. However, since it hasn't been 10 business days, or 10 calendar days for that matter, I called him back to let him know I don't think I'm legally allowed to take possession of it. I did so in a polite manner and with the intention of keeping us both within the law, but he took this as me trying to tell him how to run his business and became short-tempered, threatening to send the gun back to the sender, charging $10/day for holding onto it, and telling me that I should go turn him in to the police if I think I know better than him.
      Ultimately the discussion ended with him saying to come 10 days from today and not a day later to pick up the rifle, effectively making me waiting another 5 business days more than necessary just to spite me, though he did say he wouldn't charge me for storing the gun. At this point I'm just going to wait and pick it up on the day he specified. I just want to be done with it and make things as smooth as possible going forward.
      My question is what actions, legal or otherwise, should/could I take should things go further south with this guy? I'm mainly concerned with what to do if I go to pick it up and he refuses to transfer it to me or if he tries to charge me $10/day for him storing the gun.

<-- FFL

No idea what you could do legally but if he did charge the storage fee I can guarantee I'd cost him more than that in business by sending potential customers elsewhere through bad Facebook and Google reviews.

Your FFL said you're good to pick up your rifle and instead of picking up your rifle you told them they need to wait 10 days?

Am I reading this correctly?

Next time, pick up your gun when the FFL says "come and get it." It is not your responsibility to make sure the FFL does his job correctly.

I will refer you to the law you quoted. It specifies what the FFL must do. It doesn't say jack about the customer. It's all on him.

This!