RE: Clean barrel/dirty barrel
Some do and some don't show any signs of lost accuracy after a thorough cleaning. I believe the reason for the improvements is pitts ornon-smooth face in the bore. After some fouling and light copper they smooth these inconsistencies out therefore improving your accuracy. How they got there can be lack of cleaning or care, leaving solvents in for extended period of time, etc. I personally think more harm can come by not cleaning the bore, so clean it and if it requires a foul shot or 2 then thats what you do prior to hunting.