Didn't mention it, but I do clean the breech plug each time I clean the rifles (I have a 3mm drill bit I epoxied in a wood handle). I'm starting to think it's plastic fouling, but don't understand why I've never had a problem with other inlines (T/C Omega, Remington 700ML, Knight disc to name a few).

I usually clean with cold water (sometimes window cleaner). It seems cold water gets the BP fouling out faster.

Scope & mounts are solid (I switched to a Leupold scope I had and know is good last year on the CVA).

BH 209 is not available locally, so no chance to try it yet.