I was examining my AR-15 the other day and found a bit of oxidized copper fouling at the mouth of the muzzle and inside the flash hider. The barrel was a little fouled, but I had to run Hoppes through a few times before I started seeing hints of blue on my patches (I don't like running the heavy copper solvents through chromed barrels).

I'm not a clean freak, but it still surprised me to see that blue fouling. Any idea what was cause it to be so pronounced? I've fired Lake City SS109 and Federal American Eagle through it, maybe 400 rounds since the last field stripping. No super cheap or foreign loads.