This isn't a sales pitch, just something I found out the hard way. My guns were frustrating the heck out of me. My new one shot great but the other three just stunk. Inconsistent no matter the bullets I tried or adjustments made. The ammo was costing me a fortune to try to sort them out. I finally cleaned them with regular solvent and then scrubbed and scrubbed with copper solvent. Took forever to get a patch that didn't have any blue on it but the group sizes dropped in half or more on all three guns. I now do the scrub down with copper solvent at the end of every season. A buddy of mine had a slug gun that wouldn't group. He was ready to put a new scope on it. First, he tried the copper solvent and the groups shrunk by two thirds. Slug guns shooting sabots will get a plastic buildup that is impossible to get out without a stiff brush and a strong solvent. I didn't name any particular solvents but most any copper solvent should work.
Carburetor cleaner does a great job on plastic removal. Just be care full of your stock. I like kroil for powder removal and any good copper cleaner. Also a good bore brush is a must. Keep them clean!