Nothing........When I'm done sighting in my slug gun I leave it alone until the season is over with. Which for me is about a three week period. So, where you using foster slugs or sabots. Hard to tell since you mentioned you used "standard" Remington Slugs and then bring up the plastic in your bore. Makes me think you were using sabot slugs. I've always used Hoppe's #9after the seasons over withbut didn't have any problems.