RE: T/C Shockwaves and Pyrodex
I use the 200 grain T/C Shockwaves in my Encore and have not had any issues with "plastic" fouling. I have shot them using three 50 grain Pyrodex pellets and currently use 130 grains worth of Pyrodex pellets. The barrel gets the the typical powder fouling between shots but that is all I have noticed. I think that if your rifle shoots it's best with 150 grains, go for it. This is something you will have to find out for yourself by doing a lot of testing at the range. Don't just shoot 150 grains of BP because your rifle can handle it. Find out what load shoots the best from it.
If you do experience plastic fouling, use Mark's suggestion.