Mr. Musket,
Even with a premium barrel, the lay or direction of the finish is in the direction of the bullet travel, so fouling is minimal. This is true of any properly finish-lapped barrel regardless of how it is rifled. 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, 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 released into the gas which at this temperature and pressure is actually a plasma. This copper dust is vaporized in the gas 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 become difficult to remove. 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 reason for the "fire-one-shot-and-clean" procedure.
Each barrel will vary slightly as to how many rounds it will take to break in. This is because of things like machinability of the steel, steel chemistry, or the condition of the chambering reamer. For example a chrome moly barrel may take longer to break in than stainless steel, because it is more abrasion resistant, even though it might be the same hardness. Also, copper will stick to chrome moly better than stainless steel, so it will usually show a little more "color" if you are using a chemical cleaner. Rimfire barrels can take an extremely long time to break in . . . sometimes requiring several hundred rounds or more. But, cleaning intervals can be extended to every 25-50 rounds. The break-in procedure procedures are really the same except for the frequency of the cleanings. Remember the goal is to get or keep the barrel clean while polishing out the throat.
The best way to break-in a new barrel is to observe when the fouling becomes reduced. This is better than believing that there is a set number of "shoot and clean" cycles. Many customers report almost no fouling after the first few shots, and more break-in would be pointless. Conversely, if more "shoot and clean" cycles are required, a set number would not solve that problem either. Besides, this break-in procedure is not a completely harmless operation, so it should be done carefully and no more than necessary.
There is no hard and fast rule about barrel break-in procedure, and this is only meant to be a guide to "break-in" based on our experience. Some barrel conditions (chamber, bullet, primer, powder, pressure, velocity etc.) may require more shoot-and-clean cycles than others. It is a good idea to just observe what the barrel is telling you with its fouling pattern. Once your barrel is broken in, there is no need to continue breaking it in.
You should Initially perform the shoot-one-shot-and-clean cycle five times. If copper fouling isn't reduced, fire one shot (5 more times) and so on until fouling begins to drop off. At that point shoot three shots before cleaning and observe. If fouling is reduced, fire five shots before cleaning.
This is one of the Tech Tips on our website. Take a look for "real" answers to your shooting/reloading questions.
- Innovative
