When using some designs of scope mounts, lapping is recommended to get the best performance. Lapping can increase the amount of surface contact between the ring and scope tube, and also help with proper alignment between the scope rings.

Why would you lap scope rings?

Lapping—or truing up the inside surface of your scope rings—protects the outside of the scope tube from damage; eliminates stress on the tube that could affect the integrity of the scope's internal moving parts; and aids accuracy by removing any stresses that the scope tube could be applying to your rifle's action.

How long should lap scope rings be?

After perfect alignment rings lapped to 100% with 2 episodes within 15-20 minutes by hand-no big deal. Now scope is mounted very well without damage due to over tightening when rings are not in contact all the way around diameter of housing.