Sleeving will add weight, move balance forward, and slow swing unless the gun's curent barrel wall profile is maintained - and this can be done via striking and boring. If the sleeving is to fix too thin barrel walls, then the gun weight, balance, and swing is already not as built (guns were not built with too thin walls!). Some sleeving jobs leave the walls thick. Others have the walls struck and bored to more typical game gun thicknesses. Sleeving is a golden opportunity to have a gun adjusted the weight, balance, and swing you desire. Modern craftsmen have mostly mastered welding the joint, making it basically imvisible. They are very sufficiently skilled to make proper choking and to establish proper regulation.
Sleeved barrels are a lot like a motor overhaul - you don't have a new car, but you have the chance for a lot more miles for a lot less money than with a new car.
