What is the recommended procedure for breaking in a new barrel?
For the first ten shots we recommend, if possible, using jacketed bullets with a nitro powder load. After firing each bullet, use a good copper cleaner (one that has ammonia) to remove copper fouling in the barrel. If you look into the end of the barrel after firing a shot, you will see a light copper-colored wash in the barrel. This must be removed before firing the next shot. Somewhere in the procedure at around shot 6 or 7, it will be obvious that the copper color is no longer appearing in the barrel. Continue applications through shot 10.
If you have any ammunition left, you then may shoot two rounds and clean it for the next ten shots. This is simply insurance that the burnishing process has been completed.
In theory what you have just accomplished is the closing of the pores of the barrel metal which have been opened and exposed through the cutting and lapping procedures.
The same process may be used with firing lead bullets and black powder to do the break-in procedure with the exception that in this case you should shoot 2 bullets before cleaning for the first 30 rounds. You could use harder lead if available. This will accelerate the break-in. This will accomplish the same thing as the jacketed bullets.
After following the procedure, your barrel's interior surface will be sealed and should shoot cleaner and develop less fouling for the rest of its shooting life.