Maybe someone here can back me up or correct me on this but if you're sighting in your rifle by firing shot after shot, you're warming up the barrel. A heated barrel may not put the bullets the same spot as a cool barrel. This can change bullet impact dramatically, even at short ranges. Therefore, if you've finally got your rifle sighted in with a warm barrel and you go home then days later decide to head back to the shooting range on a beautiful day and shoot some more, those bullets shot through that cooled down barrel may not go where you expect them to go! If you're going to be hunting with this rifle, you definately want that first shot from a cool barrel to be on the spot because that's the shot that you're likely going to be firing at agame animal.

When you sight in, allow the barrel to cool down completely between shots that way every shot is like it's the first shot.

Good luck