Copper corrosion in chamber of carry piece?

Was doing a monthly cleaning of my carry piece. All was pretty good, ran a patch through the barrel for a clean coat of oil and to knock the lint, etc out. Noticed it came out a bit dirty with a touch of blue. Copper fouling? Didn't know why, I hadn't fired it lately.

I clean the barrel and then during reassemble noticed the bullet that was chambered had the green patina seen on copper. To me this means there's some corrosion going on while the bullet sits in the chamber.

I do leave a very light coat of oil (Hoppe's 9) in the barrel.

Are the copper jacket, oil and steel not playing well. Should I leave the barrel dry?

Thanks!

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Copper corrosion in chamber of carry piece?