The residue is caused by the low shear strength of the polystyrene. I haven't confirmed this myself but, I've heard that this still occurs even in overbore situations.
What would the end result you're looking for?
Have you found the residue to cause problems (i.e. velocity consistency, inaccuracy, etc)? I've only found it to be a problem in the rifled barrel which can cause enough residue (or as I call it 'fouling') to stop a round halfway through the barrel.
An intermediate step may be to find a J&J ceramic barrel that has teflon and see if that has any reduction in residue build up.