I tear down and clean every firearm I buy.
I bought a new savage axis 7-08 and the shit I washed out of that thing was horrific. Steel shot, chips and chunks galore right out of the box. It turned out a decent shooter. I was surprised.
Another was a used 1978 Browning 7mag bar that was like new but would jam the second round and not fire it. The guy fired it twice and just left it in his closet. When I bought it I disassembled it and found a disconnector spring laying loose in the bottom of the trigger housing. I had to fully disassemble the trigger to reinstall that spring, no idea how it got loose, but it runs great.
The worst (so far) is a Rem 740 I inherited with the 1954 grease packed in the action and trigger like filthy tar. After that fun I put it all together and THEN I scoped the bore. What a mess. Pitted in a few spots and copper fouling that is going to be more work to clean for probably a useless gun. But for sentimental reasons I’ll do it and go waste some 30-06 with it at the range.