I suspect the phosphoric acid can clean copper out of a barrel. I don't know whether enough copper would affect the Park solution or not? I've always plugged bores. Un-abraded metal will get a thin phosphate coating if it is clean and degreased. It won't be a lot. Perhaps a ten thousandth or so. It's not as thin as bluing. It won't wear for very long in the barrel except where it is protected in deep tool marks. My main reason for keeping it out is the gut feeling that getting even a patchy layer of something in the bore might affect the time it takes for the barrel to settle in? I've not attempted to prove that one way or the other.
I've purchased a number of Parkerized new Garand barrels in the past. I've never seen one that didn't have a shiny clean bore, so perhaps the barrel makers have the same notion?
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