In addition to revealing how clean a barrel is, a borescope can also expose machining abnormalities that can negatively affect a rifle’s accuracy.
I have a number of rifles with which I like to shoot groups. Although I clean them regularly after each trip to the range, I cannot seem to get the barrels clean enough to push a patch through and have it come out totally clean, no matter how hard I try.
One of the guys I shoot with suggested I get a quality borescope to see if I am really getting the barrels clean. Could you provide me with a few specifics as to what to look for in a truly clean barrel and in a quality borescope? He also suggested that I might not like what I was about to see aside from the fouling in the barrel. What was he getting at?
F.L. Waxman, Hartford, CT
Getting a barrel “perfectly clean” so a clean patch inserted in one end emerges from the other end in the same condition that it entered is, in reality, a pretty tall order. It can be done, but is not essential for optimal performance. As a rule, unless the barrel’s bore is completely free of solvent and/or lubricant, leaving only bare metal for the patch to contact, there will always be a discoloration to a patch—be it ever so slight—pushed through a recently cleaned bore.
A borescope tells the real story of what a barrel looks like on the inside from the chamber to the crown at the muzzle.
It should go without saying, prior to borescoping a barrel, all the loose firing residue remaining in the bore after a trip to the range should be removed with a nylon brush or a few tight-fitting patches. This eliminates seeing something with the borescope that is a non-issue regarding the job at hand. The same applies to a newly manufactured barrel to remove any preservative oils or remaining foreign particles of material left over from the manufacturing process.
An unfired barrel or a “perfectly clean” barrel will appear consistent from end to end, with no shadows or discoloration apparent to the viewer. Be prepared to see tool marks left from the reaming of the barrel, as well as the forming of the lands and grooves. It is rare to see a mirror finish in a barrel unless it has been hand lapped by a high-end barrel maker. Tool marks, depending on how pronounced they are, can be collection points for barrel fouling.
For general considerations on borescoping an unfouled barrel, start at the chamber end first. Ensure that the chamber is in line with the bore—since most barrels are chambered after the bore is finished, it is possible to find the two out of alignment. Forward of the chamber is the throat, where there should be a smooth transition without burrs or deformation from the chamber to the fully formed lands and grooves. Every land and every groove should be of the same height and depth from this point to the crown at the muzzle.
If the barrel is for a gas-operated firearm, pay close attention to the gas port to ensure that no burr was left when the port was machined into the barrel. The crown is where the bullet contacts the barrel last as it travels downrange toward the target. It is imperative that the crown be smooth and consistent around its circumference for the best results in accuracy.
Be prepared to find minor imperfections if you are thorough in your inspection. Few are significant enough to cause accuracy issues by themselves unless they are at the muzzle.
Carbon and copper deposits in a barrel are the two main detractors to accuracy in a rifle barrel. If lead bullets are being used, we can add lead to that equation.
Carbon usually appears as black streaking in the grooves and along the lands in the first half of the barrel. Carbon buildup located just in front of the cartridge-case mouth is prevalent in rimfire cartridges, but can be formed in centerfires as well. Carbon deposits have no value to accuracy and should be removed chemically, by brushing or with mild abrasives completely to preserve accuracy.
Lead deposits have a silvery appearance and can appear from the chamber to the muzzle due to more variables than space will permit fully explaining. While you’re scrubbing, a tight-fitting bore brush is usually preferred. Lead deposits can be removed in a similar fashion to carbon deposits as well.
Copper deposits in a barrel often form closer to the muzzle than carbon deposits. Visually through the borescope, they often appear as copper-colored streaking, predominantly on the lands, with heavier deposits sometimes collecting in the grooves. Copper deposits are best removed chemically, although mild abrasives will also remove copper.
When considering a borescope, you get what you pay for. Your friend’s recommendation to go for quality is correct. Optical quality is paramount for studying the inside of a barrel. A variable light source aids in the interpretation of what is being observed by controlling glare and shadowing during inspection. The ability to video and/or photograph the bore validates progress in cleaning and levels of barrel wear over time. You’re about to discover a new world with the introduction to the inside of your barrels.
