The M77 front action screw hole is a blind hole. If rifle was bedded height can be changed or some bedding material got lodged in bottom of hole. I have seen this enough to inspect every M77 and ascertain that the screw is not bottoming out.
A careful inspection of the how the rings fit into the integral bases is warranted too. It is easy to get them misaligned.
As to the barrel being worn out, a bore scope will help figure it out. Could be severe fouling. I have encountered rifles with owner only using Hoppes which leaves copper fouling so severe that there were speed bumps of copper in the bore. It took a long time with a good copper solvent to get it clean.
