From what I have seen, as a gunsmith for 32 years,the consequences of inadequate bore cleaning have more to do with long term bore damage than short term accuracy issues. If copper is left in a bore for years and years a chemical reaction occurs between the copper and the steel underneath. This reaction causes pitting. And if the barrel is eventually cleaned thoroughly, the gun usually will shoot verypoorly until enough copper fills in the pits. Personally, I clean my barrels down to zero copper and then coat them with Sheath until next hunting season. But those who don't are part of my job security