And make sure the front one is snug. I had an issue with a rifle with the zero jumping up and down for elevation.... drove me crazy till I figured out the front ring was loose. Was already at the deer camp at the time, so the solution was a shim made out of two thicknesses of grocery-store receipt paper.
Got a deer the next day and those paper shims are still in it!
Got a deer the next day and those paper shims are still in it!
