The Shooter's Choice product for shotguns and choke tubes sounds like a good choice if available. Acetone, or MEK should take the majority out, or at least soften it so that a brush or abrasive paste isn't necessary. These strong solvents need to be handled with care, but should be available in local hardware stores.
When using a bronze brush, be sure to push it all the way through, meaning with breech plug removed, or else risk "brush cutting" when reversing direction of the brush.
I've not seen plastic fouling in my Omega, and can't say that I've felt it in my Grey Hawk. That isn't to say that it doesn't happen, but the modern sabots must be working well as formulated.