It is usually a question of how smooth the bore is. A top quality barrel, that is not worn much will usually put the first shot in the group. A barrel that is rough, either manufactured that way, or has been shot a lot, can take quite a few shots to settle down. I have seen many overbore chamberings, such as the 22-250, take 15 shots to settle down when they are burnt(alligator hide through a borescope) pretty good. They still have good accuracy once settled, but long before that point I loose patience and rebarrel.
Jack