I use CRC 5-56 Multi-Purpose (similar product to WD40) to remove plastic wad residue in shotguns. Running a bronze brush followed by a patch through shotgun bores, even chrome lined bores, leaves them looking clean and shiny but spray CRC 5-56 down the bores and then look and you will see the plastic residue lifting from the bore surface. This 'dissolved' residue can be scrubbed out with brush and patch leaving perfectly clean bores.
While CRC 5-56 does affect certain plastics I use it to clean all surfaces of firearms including wood and plastic stocks and have found it does not affect any of the usual firearm finishes. Combined with fine steel wool it removes rust from blued surfaces without affecting the bluing.

CRC is a global entity including in the USA.