I had some kind of build up. use to shoot the old sabots but also greased them with bore butter..a combination I wouldn't recommend. At the time we didn't know you werent' supposed to "grease" the sabots, leftover habit from shooting conicals I guess. Anyway was having some acuracy issues with my Rem700 and using a light I could see some type of build up where the lands and grooves meet. cleaned with soap and hot water until patches were clean then I tried some bore solvent that supposedly was designed to remove plastic fouling and the patch came out almost solid black. cleaned with the solvent until I got clean patches then oiled it etc. next trip to the range it was shooting one ragged hole at 50+yds. I don't know if the residue was plastic, bore butter or what, but I alsways felt like it was from shooting those old plastic sabots.