I've been told that during multiple shooting sessions without decoppering a bore, it gets layered with copper covered with powder residue and then during the next use copper builds up on the residue and gets covered with more residue. So when you finally decopper the barrel, you litereally are peeling off layers of copper.
I can't prove that it happens but is sure would explain why after a first cleaning you might have noticed a significant second dose of copper on the next cleaning.
+1 for Wipe Out foam for calibers over .224. It has the added benefit of not needing additional cleaning after its use.
For .223 calibers, the foam can nipple doesn't fit into a .224 bore and goes the foam all over the chamber and action when I tried it.
Now I use the Patch Out fluid on patches for my .223 and .22-250 and it still works the Wipe out miracles. Even with the foam, a patch of Wipe Out Accelerator fluid makes it decopper even better in about half the time.