Regardless if it is a standard twist or a gain twist you are NEVER EVER supposed to lap a RKS barrel.

For accuracy it has a continuous precision choke of .0004" from the chamber to the muzzle.

Lapping will result in variations or change the degree of angle of the choking or interupt the continuity of the perfect slopped choke Ron has installed. 
Doing so will reduce your accuracy by affecting the perfect precision choke he has already installed in the rifle.

It is also strongly recommended to NEVER EVER use jacketed bullets in a rifle solely intended to shoot cast bullets since copper fouling cannot be readily and completely removed and may significantly affect accuracy when you start using cast bullets (some of the time).