It all depends on how smooth the bore is prior to nitriding. If it's a lapped custom barrel, no break-in is required. Even if it's a good factory barrel that doesn't have a lot of tool marks, it's good to go as-is. The reason it might foul more in some cases is because the process case hardens the surface of the bore just like it does to the exterior, so it takes longer for the bore to smooth out from firing.
