My thought when I read this last night was "copper fouled". I bought a 30-06 from some bubbas a while back for a song. I knew they were selling it cheap because it wouldn't group. I cleaned that barrel with Hoppe's copper cleaner for two weeks and it's now one of my most accurate rifles.
X2
I've had the same thing happen. A barrel can look clean, just looking down the bore, after regular cleaning with a quick patch and brush cleaning, but still be copper fouled.
