Bushnell Banner Scope Dissasembly

My Bushnell Banner 3-9x-40mm scope has a zero that moves under recoil up and down about 1 moa. I am trying to use it for F class competition and the zero change just ruins my score.
I want to take the scope apart so I can see if there is anything I can do for it but I can't see how to get the rear bell off. It unscrews to a certain point then stops. There is a small hole exposed when the bell is at the rear most point. Not sure if this has anything to do with dissasembly or not. Anyone here know how to take these apart?

I've got a Banner I bought in the late 70's and had a similar problem in the mid 90's. I sent it to Bushnell and they fixed it free. You should call them and see if they'll cover it under the lifetime warranty. http://www.bushnell.com/global/customer-service