What I see with a magnified bore scope,
Several burrs, pits in the barrel.
Rifling is a drag through, high pressure machining process leaving behind issues.
What I see with full power ammo straight away is copper being scraped off into pits.
Pits enlarge when full power rounds are pounded through it.
Copper become plastic under that pressure and expands defects.
Keep in mind a pit was a defect in the steel to begin with, usually a 'Nodule' or a tiny speck of reactive substance. The copper flow simply expands that defect to it's margins.
What I see is burrs/shavings left behind imbedded in the barrel.
I see small, but jagged edges left where burrs are broken off.
I see scratches/gouges where steel has been broken off and dragged down the barrel.
Keep in mind, this isn't speculation or theroy, it's direct observation.
It's repeatable simply by using a bore scope, or watching some of the videos of bore scoping.
What I see when the copper from factory test fire is cleaned out (just new barrels don't get test fired), is small defects in common, mass produced steel bar stock, that is made into a barrel.
Anyone that has ever seen a magnified picture of steel can pick out defects, steel production isn't exactly a 'Sterile' process, contaminants do get in, minerals do clump (nodules), there is no way to keep it out entirely.
Dragging a broach or button down the barrel imbeds impurities, so does a hammer forged mandrel.
Just a side effect of the rough machining processes needed to form rifling.
With the best cleaning the barrel's life to date, simply remove 100% of the copper and back/forth motion of cleaning brush will break loose many of the burrs before you fire anything through it.
High end barrels get an abrasive compound (very fine grit) and a rifling fitted 'Jig' (or plug/jag) working down the sharp spots where burrs broke off, smoothing down the sharper edges of pits so they don't scrape off copper and cause hydraulic expansion of those pits/defects.
This process is called 'Lapping'.
Lapping also gives you an extremely CONSISTENT bore/rifling diameter.
Machining produces a pressure wave in the material ahead of the tool, which 'Rebounds' when the tool passes.
This pressure wave/swell/rebound will happen in ANY material when something displaces it, from air to water to steel. You have all seen the bow wave in front of a boat, and the rebound swell behind a boat, same thing in air which you can't see but feel on the highway when passing a big truck, and steel is no different. Steel being much more dense, the swell/rebound is very small, but it's there.
Lapping helps even out these 'Warps' or 'Ripples' (stress waves) in the steel, uniforming the rifling by removing the 'High' spots.
Once VERY clean, you can do somewhat the same process (to a much lesser extent) by firing lower powered/pressure rounds.
The issue is, even though you are reducing pressure that will cause hydraulic damage, you are still using copper jacketed bullet, which will still leave behind copper pressed into every defect in the barrel.
The ONLY way to get that left behind copper out is clean again, and again, and again, and again...
Clean with a high powered copper reactive cleaner, something that corrodes (and in the corrosion process loosens) & eventually removes copper.
Shoot, clean, shoot, clean, shoot, clean...
After the first 10 to 20 shoot-clean cycles, what I see is MUCH less copper being scraped off into pits or by sharp edges.
The friction of the bullet rounds over edges, both reducing the amount of copper that gets gouged off the bullet, but also smoothing & rounding edges (much like lapping) does.
Pits get larger, margin to margin, but it's a slope into the pit center, then a slope out of the pit center.
It becomes a 'Dip, rather than a square edge hole.
The copper didn't get a chance to build up in the center of the pit, then the high pressure from the next round blows out the pit even deeper, instead the edges are smoothed round, pit becomes a 'Dip' so copper left behind gets pushed out forward of the next bullet, pressure relief being the key to NOT causing a hydraulic pocket.
We all know it takes several rounds for a barrel to 'Settle Down' or 'Break In' and shoot consistantly.
Molicules in the steel alloy are finding their places as the pressure wave from firing passes through them, literally 'Settling In', tight spots getting clearance, as the rifle is fired...
This again is indisputable, it's scientific fact which every precision shooter has directly observed.
By the time you get the first 50-100 rounds through a barrel, the chemical/manual (brush) cleaning isn't a huge issue (unless you are REALLY lazy), most of us have cleaning down to a fine art so it's neither difficult or unfamiliar, and it ensures a good shooting barrel for several thousand rounds in the future.
Reloaders have zero issues producing 20 or 30 lower power rounds for initial break in, and there are a few places that produce lower pressure break in rounds if you don't reload.
At the very worst, the barrel gets 20 great cleaning and wastes your time... An at the very best ensures your new barrel/rifle is as CONSISTANT as it POSSABLY can be.
You all argue amongst yourselves, quote magazine articles, speculate, theorize, regurgitate the same old dogma, I'll go with first hand observations and good to excellent results, scientific based research and continue to lap & break in barrels for a (relatively) super smooth finish that doesn't have copper issues.