Unless I had a serious blockage in the barrel I would not attempt to remove the breechplug on a sidelock wildeboer. That's a procedure that easily (and often) comes to grief. It's really not necessary for a good cleaning of a sidelock. Plastic fouling from sabots in a sidelock is highly unlikely. But if you suspect such flush the bore with boiling water. Any plastic in there will come loose and easily wipe out with a patch while the barrel is very hot. You may want to wear gloves to handle the barrel while doing this.