I' m having a problem with my clay loads. When I clean my barrel after shooting, I get a black plastic coating the entire length of my barrel. Looking down the barrel, before being cleaned, it looks good. After I run a patch soaked with Hoppes No. 9 through it, a bunch of black plastic stuff gobs up through the whole barrel. Its quite a mess to clean. Why is this happening?

My loads are Rem STS hull
WAA12 wad
1 1/8 oz #7 1/2
International Clays powder using #29 bushing in a Sizemaster
and 209 Win primers.

I think the gunk is coming from the wads. They look " smeared" after being fired.

Thanks for any help!!!