BA is right about break-in being about removing reamer marks from the throat, and not to somehow improve the bore or rifling.
If it's a stainless match barrel follow the manufacturer's break-in procedure. If done right, you will have polished the throat, have removed the copper buildup that tries to attach to those reamer tool marks, and will get much longer time between bore cleaning.
Here is the method I used on my Lilja Recce barrel.
https://riflebarrels.com/support/centerfire-maintenance/This is a sub 1/2 MOA match barrel made to benchrest standards. I doubt the break-in improved accuracy, but the barrel has very little copper fouling now and cleaning is easy and infrequent. It allows the barrel's inherent accuracy potential to be reached and maintained longer before cleaning is need again (when groups start to open up).
If its not a match grade barrel shooting match ammo, don't bother. Just shoot it. Same with a nitride barrel. Nitride is going to be too hard to successfully remove the reamer marks.