No. I don't think you have to say anything about what u have. And air rifle is not a weapon. They are pretty clear on what u can or can't have. Weapons is a big no no. Since a bat could be a weapon. A hammer could be a weapon. If u admit you got "weapons" means u are carrying prohibit devices. And unless they have a search warrant. Don't let them look inside your car. Nor do u have to tell them what's in the trunk. If they ensist a search. Walk out of the car and lock the car then call a lawyer.
This has been frequently brought up on gun forum. Even real guns are not weapon. If you admit your firearms or air gun ( or your tactical spork) are weapons. They might hold you in for trouble.