from BallisticStudies:
Sometimes, a rifle barrel may have a burr on the rifling land edge, seen as long flake. If the burr is thick, hand lapping operations will fail to break it away from the land.
Worse still, if the rifle has been fired several times, the burr becomes forged into a lump type anomaly, wedged into a groove
Ultimately, the end user has final control over the finish of the bore. If the bore is of a high finish, the bore will need nothing more than normal cleaning/de-fouling operations. If the bore is rough, it will need final finishing by hand which for want a better term is called breaking in the barrel.
Kriegerbarrels:
If it is not finish-lapped, there will be reamer marks left in the bore that are directly across the direction of the bullet travel. This occurs even in a button-rifled barrel as the button cannot completely iron out these reamer marks.
Because the lay of the finish is in the direction of the bullet travel, very little is done to the bore during break-in, but the throat is another story. When your barrel is chambered, by necessity there are reamer marks left in the throat that are across the lands, i.e. across the direction of the bullet travel. In a new barrel they are very distinct; much like the teeth on a very fine file.
When the bullet is forced into the throat, copper dust is removed from the jacket material and released into the gas which at this temperature and pressure is actually a plasma. The copper dust is vaporized in this plasma and is carried down the barrel. As the gas expands and cools, the copper comes out of suspension and is deposited in the bore. This makes it appear as if the source of the fouling is the bore when it is actually for the most part the new throat.
If this copper is allowed to stay in the bore, and subsequent bullets and deposits are fired over it, copper which adheres well to itself, will build up quickly and may be difficult to remove later. So when we break in a barrel, our goal is to get the throat polished without allowing copper to build up in the bore. This is the reasoning for the fire-one-shot-and-clean procedure.
Every barrel will vary slightly in how many rounds they take to break in For example a chrome moly barrel may take longer to break in than stainless steel because it is more abrasion resistant even though it is a similar hardness. Also chrome moly has a little more of an affinity for copper than stainless steel so it will usually show a little more color if you are using a chemical cleaner. Rim Fire barrels can take an extremely long time to break in, sometimes requiring several hundred rounds or more. But cleaning can be lengthened to every 25-50 rounds. The break-in procedure and the cleaning procedure are really the same except for the frequency. Remember the goal is to get or keep the barrel clean while breaking in the throat with bullets being fired over it.
Finally, the best way to tell if the barrel is broken in is to observe the patches; i.e. when the fouling is reduced. This is better than some set number of cycles of shoot and clean as many owners report practically no fouling after the first few shots, and more break-in would be pointless. Conversely, if more is required, a set number would not address that either. Besides, cleaning is not a completely benign procedure so it should be done carefully and no more than necessary.
Cabelas:
Don't be in too much of a rush. Plan on investing some serious time in breaking in, or seasoning, the barrel on that first outing. That is, if you want to extend the life of the rifle and increase its potential for consistent accuracy.
The breaking-in process is important because it will smooth any imperfections left in the bore from the manufacturing process, according to Cabela's gunsmith David Orten. If not dealt with properly,
the imperfections can adversely affect your rifle's performance.
me:
I've also heard it described as similar to a washboard road. I tooling mark or burr forces the bullet to the side creating a an uneven wear spot. The previous wear spot creates a following wear spot etc.
Whether truth or myth I don't want to take the chance of killing the accuracy because I didn't think a break in period was needed. Besides it means I get to shoot more and I like to shoot.
