Years ago I had a similar problem. In my case the plastic had been deposited over time shooting normal loads. When I shot a round of trap using BP loads it "semi-liberated" that plastic buildup on the shotguns bores. I used a 12 gauge bore brush with a stout aluminum cleaning rod spun in the bore with my power drill. Even with that it took about 45 minutes to clear both bores.
BTW you might try plugging the barrels on one end and then filling the bores with acetone and let sit for a few hours. Just be careful not to get it on your guns wood finish.
Steve
