Ever use J-B compound? When used per the instructions, this is good stuff. I have a .270 that I've owned for about 30 years now. I always clean my guns after shooting. This rifle shot great, really great, for 20 years with Nosler bullets. Then I started to notice a gradual decline in accuracy. I used the J-B per the instructions and even though the bore looked clean, the junk that came out of it was incredible. The old rifle is back to shooting nice little clusters of Nosler Ballistic Tips.
From what I understand, those ultra-sonic cleaners do a good job too.