That build-up was hard and I was worried that I would be taking metal before.
This is a well known problem and isnt just BH209 specific. Long term use of 209 primers will leave a hard deposit if not cleaned properly after use. Many people just assume its the metal. Mags leave more carbon in the channel than non mag primers.
The same thing happened to me with my first Knight Rifle. I assumed the channel was clean and it wasnt.
Is there something to the comment earlier about a breech plug being modified for BH209?
The typical modification is made by enlarging the flash channel to 5/32 and replacing the flash HOLE with a Savage type vent liner. Sometimes the vent liner is slightly recessed in the breach plug to create a small "pocket" for the powder. It also brings the powder a little closer to the primer.
Vent liners are around $3-5 each and you simply replace them when the hole reaches .035 or more. It beats the heck out of buying a new plug. The larger flash channel takes longer to fill with carbon and reduces pressure to the primer when fired.
Plug on left has the vent liner mod
It looks like this from the top.
