If your sure it is plastic meltdown from the sabot, then bore butter is not going to solve that. You need to change sabots. You did not mention what kind of sabot your shooting, but I have shot Thompson Center Mag Express sabots with 120 grains of powder and never had any problem. I will guess that your shooting pellets and not loose powder. So I would loose the pellets, and change sabots to something else. Swab between shots and I think your problems will disappear. If need be, you can use a bore brush and scrub the bore between shots.