If a hmr, or centrefire rifle is used in damp weather like is typical night time shooting this time of year, if the residue isn't removed from the inside of the barrel it will attract water ------rust----then pitting, for me the minimum after use is two dry patches followed by a lightly oiled patch, or better still 8 strokes with a bronze brush with white sprit, two dry patches followed by a lightly oiled patch, -------i'm not a believer in copper cleaning as i've not found it necessary and it often takes 5 shots to get the hmr back on target after a copper clean.
Dave (warbucks)