Years ago when I went through sniper school, everybody at the school did barrel break-ins. None of the people at the sniper school would even begin to think of any barrel or rifle as being broke in until it had, at the very minimum, 500 rounds through it. The overall consensus was that barrels can take thousands upon thousands of shots before you will ruin one.
The key was to use good ammo, clean frequently and shoot slowly with the first several hundred to 300 rounds. Do not heat up the barrel with the first few hundred shots. Fire one round and pause. Fire another round and pause. Keep doing that along with periodic bore swabbing and cleaning. The most rounds that you want to shoot before cleaning or swabbing is no more than 5 at any one time.
Before you begin, get yourself a decent 5 round shot group from the barrel right as you begin to break it in. After about 300 to 350 rounds get another 5 round shot group to compare to the first group. At 400 rounds you get a third shot group and then at 500 rounds you get a fourth shot group for comparing. By the time you get your fourth shot group, you will actually see a very noticeable difference between the 1st shot group and the fourth shot group. Often you will see a group come out of the barrel of a rifle that is the size of about a man's palm or about 3 to 4 inches across. By the time you hit 500 rounds, the group drops down in size to about the size of a nickle or even smaller. Once I learned that breaking in a rifle barrel is a good thing, I've done some form of it ever since to include doing it for my AR-15s and M-4 clones.
Next time you get a new rifle, try it out. In addition to breaking in the barrel, the slow fire initial shooting allows other parts in your rifle or carbine, especially in the ARs and M-4 clones, to mesh together better or smooth out.
