ko4nrbs

I do not know if you use a BP anit-sieze grease or not - but back when I was I tried both a copper based and an aluminum based grease - like ;ee said - after doing that it took me forever to get the copper and aluminum bits out of the barrel down near the breech area. The stuff was shot right into the pores of the metal... I did come out eventually as I continued to shoot and clean...

Again I do not if this gives you any answers...

Plastic fouling: It should appear as black flakes usually caught behind the riflings (spelling). If you have shot shot guns you have seen this black fluffy stuff shot out of the chokeevery once in awhile.

Pouring boiling water down the barrel will fluff it up and release it so that you can swab it out.... but considering what you have already tried I am doubting plastic fouling...