The barrel is the most important part of the equation.
Factory barrels are usually rough as hell inside. Chips dragged across the lands, pilot marks in the throat, lands that have a rough surface. Savages are appalling to look at with a borescope.
Coppering the piss out of a rough barrel like that actually makes them shoot better, up until you hit a round count where the bore is getting tight. Hence the current school of thought of not cleaning until accuracy falls off. Break in provides little benefit, as that copper is actually making your bearing surfaces more consistent.
A hand lapped shilen, bartlein, krieger, etc. is usually chambered in a professional manner so as to not leave any marks in the throat, and the lands are near flawless when viewed with a borescope. There are little to no voids to fill in with copper. These barrels like more regular cleaning to shoot at maximum accuracy. And break in is beneficial to lay down a little copper, without having it build up too much.
Of course these barrels are much more consistent and accurate than any factory barrel could hope to be.