I picked up a Winchester Model 70 Stainless Steel Classic BOSS in 7 mm Remington Mag, from Gander Mountain (via internet). When I took it apart for cleaning, it looked like it had spent the winter under the ice behind a beaver dam. Whoever took it to Gander for sale, must have just hosed it off and wiped it down with paper towel. Took a good week of cleaning to get the patches to run through it clean.

Then there were the Mini-14's.... Lots of carbon/gunpowder/copper buildup. Days of scrubbing and soaking.

Most, but not all, of my used gun purchases have been cleaning nightmares. But all cleaned up nicely. Probably better than when they left the factory. I have taken to putting a tag on each one with the date it was cleaned/oiled. Memory gets a bit fuzzy when you're dealing with more than a couple firearms.

I buy patches in 1000 bag increments, as I'm a bit OCD about cleaning guns.