It looks to me like the barrel is touching the breech face at the lower right in the next to last photo. Blacken the breech end of the barrels and the surface that meets the water table.
I've used inletting black, candle black and medium and fine grit stones to very carefully stone the high spots. I'm certainly not an expert but I've built a number of nice flintlocks and restocked and rehabbed a few nice old SxSs using classic gunmaker techniques.