First off, hope it's RCMP. They're usually pretty slack about airsoft.
I've been pulled over 4 times, once we even had simulated mortars in the back and still had our facepaint on....they didn't search the vehicle that time for whatever reason....
Anyway, each time I declared I was on my way/coming back from airsoft, and the location of the event (since RCMP should ALWAYS be notified where you're playing airsoft). Only twice they asked if they could see a gun, so obviously I pull out the most non-threating gun I have, my VSR-10!
So warn them that it LOOKS real. Open the case. WAIT FOR THEIR REACTION. Then prove to them it's not real.
Both times they actually saw airsoft guns, they were more interested in the rather advanced radio I had in my car to signal the guys at the event on the way there, and the chrono I had left in my cab on the way back lol
My advice is to be very simple and straight forward in your responses. Lying to a police officer IS an chargeable offense. And let's face it, if you lie and they find the guns, your gonna look a lot more conspicuous than if you just told them you had guns.