Do you have a muzzlebrake or flash suppressor mounted? I normally swab it out well with Q-tips (and Hoppe's) and use pipe cleaners to clean out the milled slots and or holes or what have you. I see this a lot on A2 style flash suppressors after shooting CMP matches in which I fire at least 88 rounds. And sometimes it does seem like it takes forever to clean the barrel.
A perfectly clean bore can look dirty because the end attachment is still dirty and is larger than the i.d. of the bore, thus getting the patch dirty at the very end of the stroke....I hope that this helps.
A perfectly clean bore can look dirty because the end attachment is still dirty and is larger than the i.d. of the bore, thus getting the patch dirty at the very end of the stroke....I hope that this helps.