Re: Is barrel break-in really needed for factory guns?
I can certainly understand how a copper jacket bullet smoothing out the (steel) throat area of a chamber does not make sense. I am not a metallurgist, but do have enough of a science education to understand what they were telling me might be happening on a molecular level. The following is the opinion of several experts in the field of metallurgy and mechanical engineering. (And if I am preaching to the choir, please forgive my blunder)
It has been well established the chambering of a barrel leaves marks or ridges if you will, that are transverse to the passage of the bullet. These ridges, especially early in the barrels life will scrape a small amount of copper from the bullet as it passes over them. Just after the bullet passes over these grooves, if we were able to take a high speed picture of them, we would see copper in the valleys of these grooves, much like what would happen if you scraped a bullet over a file. But the copper does not stay there long. As the temperature and pressure rise in the throat area (from the burning powder) the copper vaporizes and is carried down the barrel and deposited.
The very tips of the ridges are now subjected to heat that raises the temperature to the point the metal becomes relatively soft. As the burning powder scrapes across these superheated ridges it removes the very top of them. With the next round there is slightly less of a ridge present and hence less copper and steel is removed. Sharp pointed ridges with narrow bases, from a sharp reamer tend to be worn down quickly. Rounded, broad based ridges from a dull reamer wear more slowly and produce more copper fouling.
You can demonstrate this quite well with an old file. Take a torch and play it across the teeth. You can very quickly get the teeth to the melting point while the backbone remains relatively cool.
At least in my mind this explains why custom chambers made with sharp reamers take few rounds to “break in” and deposit little copper in the process. Also that (some) factory barrels may require many more rounds to “break in”.
Krieger is again recommending the 1 shot, clean. 1 shot, clean..... 5 shots, clean......etc. I guess its not “bull” anymore. LOL