I'm no expert in bore technology but I'd imagine that the heat generated by the friction of the bullet racing down the bore makes it get hotter as it progresses. I am very familiar with copper as a metal however and it certainly gets softer as it gets hotter. Softer copper rubs off easier. Add to that, if the barrel is lapped using conventional methods, the lapping slurry tends to get less aggressive the further down the barrel it progresses from the breech, removing less steel. That's actually a desirable property in 22 caliber BR barrels. They choke down slightly as you near the muzzle, which improves accuracy. I too have seen the buildup get worse, the closer to the muzzle in my 6mm XC barrel, having gotten it after I got a borescope. It now has about 300 rounds through it and the rate of buildup from shooting nude bullets has not gotten much better, despite it having been lapped at the manufacturer. Its pretty smooth compared to some barrels I've gotten, that claim to have been lapped. So the heat factor may be a greater contributor. Lately, I've taken to shooting bullets in it that have been tumble plated with HBN. Using MoS2 or WS2 has the same effect. Switching to plated bullets stopped the copper buildup instantly. I have not seen the use of a bronze bore brush when cleaning, have a significant impact upon removing thicker copper buildup. It definitely knocks out the carbon buildup however, with just a few a passes, using a good solvent. While a few patches of KG-12 copper remover, after patching out any other chemical you were using to clean the bore, with a strong degreaser like trichlorethylene, knocks out the lesser copper buildup, it doesn't get the thick streaks out near the muzzle. That takes some elbow grease using JB compound diluted with Kroil. That definitely get the bore down to just the steel. That's what I did prior to switching to HBN plated bullets. No more copper. From my own experience, it takes more than 20 rounds to break in most barrels, unless you're using Tubb Final Finish or Neco Bore Treatment abrasive type bullets. While I don't use those, I've read that they get the job done faster. As always, YMMV.
Hoot