My guess is, that if you are using 777 you are experiencing the "crud ring" rather than melted plastic.

I've been using 777 with sabots for about two years now, occaisionally powerbelts too, and see this every time. What I do is use a T/C 250 grain sabot for the first shot and then a power belt for the second. Inside 100 yards they are only a few inches apart and the power belt can be loaded after a shot where as the SST sabots are so tight I don't think there is any way you could load one in a dirty barrel.

You should be able to clean the barrel well enuff after two or three spit patches, and then a good dry one to load another sabot though. After 4 or 5 sabots, you might need some plastic dissolver, there are several brands of this on the market.

I want to experiment with using 3 pellets and a lubed wad under the bullet. I know the lubed wad would cut down on crud, but am hesitant to use three pellets.
R
Hank