weaver grandslam focus problems

[thirty06]

A friend of mine has a 3-10x40 weaver grandslam which he is really pleased with but for the fact that he adjusts the focus to just right and after a couple of weeks of use he finds that the focus has shifted. It drive him crazy but the problen is there is no way I can find a lock ring to stop the focus from shifting while we are using it onthe hill.

Does anyone have the answer to this one?

Hamish