https://im1.shutterfly.com/med...0/rx%3D550/ry%3D400/
AR 15 barrel crown & bolt pitting. The CM barrel bore, including throat & gas port look good (normal wear). I haven't kept an extremely accurate round count, but would guess 2000-2500 rounds loaded with CFE223/CCI400. Several hundred rounds of Tulammo/Wolf/Monarch spread out within the CFE round count. Nearly nonexistent detailed cleaning, only occasional spray/lube to keep it running. I would guess some of the cheap steel case ammo had corrosive components or the carbon build up, which was heavy on the crown and bolt, had a hygroscopic effect which lead to the corrosion. The bore had virtually no copper build up and the only carbon build up in the bore was at the last two inches, and it was only streaks. Carbon build up in lower, and on/in the bolt was extensive. Yeah, I know proper cleaning could have prevented the pitting but I was curious how long I could go with out doing a through cleaning. So, what do you think caused the pitting (excluding lack of cleaning)?

If you read some of the other posts you will find that just because carbon deposits are visible & it does not mean that you do not have any copper fouling. The copper & carbon get layerd multiple time one on top of the other.

If you used carburetor cleaner to clean the carbon & then Wipeout with Accelerator, you would know for sure if you get green copper stuff on the patch. If you do get the green stuf,, you will have do prolonged multiple cleans of Carb cleaner & Wipeout to get all of it out. Plan on even up to 6 or more alternating passes with some over night soaking.

Regarding the pitting, The muzzle part depend on whether you had a threaded cap or not.

Common causes are corrosive primers, use of ammonia based solvent and not nutralising it (very common) and sea water / air.

JMTBW

Welll, 2500 rounds pretty much solves that problem, with that kind of care the bore has pitted for what ever reason but it makes little difference, the bore is shot out or pretty close to shot out for any hi vel 22, particularly in a auto, you got your moneys worth so time to rebarrel it...