Every jacketed bullet leaves a small amount of jacket metal smeared along the bore. Left alone, that deposit grows, and after enough rounds the bore carries a thin copper plating that changes both friction and the way the bullet engraves the rifling. Copper fouling is one of the two fouling types every rifle owner has to manage, the other being carbon from the powder, and it is the one that quietly degrades precision on barrels that are otherwise still perfectly good. Knowing how it accumulates, which bores foul fastest, and how aggressively to clean it will save you both accuracy and barrel life.
As a bullet travels down the barrel, its gilding-metal jacket is squeezed into the rifling under enormous pressure. Microscopic roughness on the steel, especially in a new barrel that has not been smoothed by firing, scrapes copper off the jacket and bonds it to the bore surface. Each subsequent shot passes over those deposits, and with heat and pressure the layer thickens unevenly. The throat fouls first and heaviest, because that is where the bullet is being engraved and where gas temperatures are highest.
The process is self-reinforcing. Copper does not plate onto clean steel evenly; it prefers to build on existing copper, so a bore that starts fouling quickly tends to keep fouling quickly until it is stripped back. This is one reason a proper break-in for a new barrel has some value despite the ritual arguments around it: the goal is to remove early copper deposits before they seed a permanently dirty bore. What the evidence does and does not say about break-in procedures is covered in our break-in piece.
Four factors dominate. First, bore condition: a rough or worn throat fouls far faster than a polished one, and a rebarreled rifle that starts coppering badly within fifty rounds usually has a throat finish worth discussing with your gunsmith. Second, velocity: fast cartridges with light-for-calibre bullets engrave harder and run hotter, so a .270-class rifle fouls noticeably quicker than a mid-range .308-class round. Third, bullet construction: thin-jacketed match bullets and some bonded designs shed more copper than tough conventional jackets. Fourth, cleaning history: bores that have been aggressively scrubbed with harsh brushes and poor solvents carry micro-scratches that grab copper.
If your rifle is losing accuracy well before its expected round count, copper fouling is a more likely culprit than actual throat erosion, and it costs a couple of hours to rule out rather than a four-figure rebarrel. The difference between the two, and the honest round-count expectations by cartridge class, is in our article on diagnosing a dying barrel.
A fouled bore is not a smooth bore. The copper layer varies in thickness around and along the bore, so each bullet meets a slightly different resistance pattern. The practical symptoms are recognizable: groups open slowly over a few hundred rounds, and the classic tell is a rifle that shoots its first two or three shots from a clean bore into a different point of impact than the rest of a string, because the fouling condition has shifted from clean to equilibrium. Some barrels genuinely prefer a lightly fouled state and lose their first-shot or first-few-shot consistency when spotless; that is normal, and it is not an excuse to never clean.
Two hundred rounds is a sensible interval at which to inspect for copper on a sporting rifle. Benchrest shooters working with premium barrels often go far longer between full copper removal; high-volume shooters with fast cartridges may need it sooner. Judge by results: if your cold bore has drifted or groups have grown by twenty percent, look at the bore before blaming the scope.
Modern ammonia-based copper solvents work, and the procedure matters more than the brand. Run a wet patch through, let it dwell the time the manufacturer states, and follow with dry patches. Blue patches mean copper is being lifted; keep going in short sessions until the patches come out clean or light grey. Use a bore guide so the rod does not chew the throat, use a one-piece coated rod, and brush no more than needed.
The two ways shooters wreck bores are patience and chemistry. Ammonia solvents left to dwell overnight, or several aggressive sessions back to back, can pit a bore, particularly at the throat. Never mix solvent chemistries in the bore, and finish every cleaning session by drying the bore and leaving it lightly oiled so the next cold shot does not fire over residual solvent. A barrel that has been over-cleaned with ammonia damage is not recoverable; that is a rebarrel, and the trade-offs of ordering fresh steel are in our barrel life guide.
For most sporting rifles: wipe the bore after each range session with a couple of solvent patches to remove carbon, dry it, and oil it lightly. Every couple of hundred rounds, run one full copper-removal session with patches and solvent, then shoot the rifle and re-zero if needed. Keep a small log of round count and group size. The log is what turns a vague sense that the rifle "is not shooting like it used to" into a decision you can actually make, instead of chasing scopes, loads, and bedding when the answer is sitting in the throat as a few microns of copper.