Get your rifle shooting well right from the start with proper care.
Even hand-lapped barrels will have tool marks in the leade/throat area, and the proper break-in process can eliminate them for top accuracy.
When you purchase a new car, a prudent owner doesn't go out on the highway and drive 70 mph right off the bat. The owner's manual suggests varying speeds over the first 500 miles of life. Logic indicates that you shouldn't do this to your new barrel either; that is, go buy some full-power loads and just shoot the heck out of it.
All new barrels exhibit tool marks in the leade/throat area. If it is a factory barrel it can also contain any or all of the following: tool marks the full length of the rifling, smudge marks on the top and sides of the lands or grooves from a dull reamer, voids, depressions, high spots, tight spots, burrs in the crown, variable land heights, off-center chambering and more.
If you have a new custom hand-lapped barrel, the barrel maker took great pains to lap away virtually all the tool marks. He likely poured a lead lap into the bore and worked it back and forth, applying an abrasive polishing compound on the lap every so many strokes. Hand lapping actually puts a two-directional finish on the barrel's interior (pushing and pulling).
Your gunsmith chambered this hand-lapped barrel with his (hopefully) sharp floating piloted reamer. He likely even polished the case-body portion of the chamber in order to uniform and smooth it, which will aid in extraction of fired cases.
However, he did not attempt to polish the leade area of the chamber, and if he used a solid pilot reamer, there will also be some slight tool marks in front of the leade/throat area on top of the rifling.
Every newly chambered barrel has tool marks that need to be smoothed out before the rifle will begin to shoot its best groups, and this is the area I focus on for barrel break-in.
Many recommend the tedious process of shoot-and-clean, shoot-and-clean and then shoot-and-clean some more. This is done in the hopes that sending a bullet down a clean bore will burnish away all the remaining tool marks. It helps and can work, but that process is going to take a while, and I've seen barrels that still had tool marks even after several hundred rounds.
For years now I have been using bullets coated with carefully selected abrasive grits to use for break-in, and I sell a product called FinalFinish that was developed to improve the internal condition of factory barrels through the use of a series of abrasive compounds.
Through these years of product testing and development, I have concluded that abrasive bullets, chosen and used correctly, produce optimum results in all barrels, whether they are factory or custom, but I want to focus on break-in and subsequent care of the custom hand-lapped barrel.
The break-in process for hand-lapped barrels is simple. I fire 10 abrasive-coated bullets, clean the barrel, and then fire 10 plain bullets that give a burnishing effect. This procedure removes virtually all traces of imperfections that resulted from chambering. It also "feathers" the full length of the bore and essentially perfects its finish. When I say "feathers," think of the feathers on a bird and how they all lay the same direction for aerodynamic flight. Barrels treated in this manner shoot their very best right from the start, foul less and clean up much easier.
After several hundred rounds, it's likely that you'll have removed the tool marks from the leade/ throat area no matter what was done for break-in. However, during this same period the barrel is now exhibiting a stress-cracking presence in the throat and down the barrel a few inches. This is normal. There could be cleaning rod wear marks appearing in the rifling too.
A roughened throat compromises the bullet jacket's integrity as it travels down the barrel. When the throat becomes rough enough to tear or mar the bullet jacket, the barrel's accuracy will deteriorate.
After several hundred rounds, you need to think about some sort of scheduled maintenance on the roughened throat. This is akin to changing the oil on the car that you broke in correctly and is now being run at high speeds.
Using the same abrasive-coated bullets, I fire two to three of them through the rifle to smooth this area, which will keep it smooth. My experience has shown this procedure increases accurate barrel life by some 30 percent.
Many people wonder whether the use of abrasive bullets is going to "push" or extend the throat on their rifle.
I recently shot a brand-new Gary Schneider polygonal 6mm five-groove barrel with a 3/4-degree leade. I measured the newly cut chamber's leade/throat with a dummy round loaded with a 115-grain DTAC bullet, seated into the rifling. I then fired 10 abrasive bullets through the barrel and measured the throat advancement in the same manner.
It had advanced only 0.003 inches. Basically nothing changed except that the tool marks in the leade/throat area were gone.