In addition to breech plug scrapers, there are wire brushes that can be used to scour the inside of dirty breech plugs.
Here's what the Investarms manual has to say regarding cleaning and breech plug removal:
Of course, the muzzle loading firearm can be cleaned
without any dissassembly but care should be taken
to prevent water and excessive solvents from entering
the stock or lock mechanism.
PROJECTILE NOT SEATED AGAINST THE CHARGE: if,
due to powder fouling or other circumstances, a ball
becomes lodged part way down barrel, then the
rifle must be disassembled and the charge removed.
With the proper tools, the breech plug can be
removed from the barrel. Consult a gunsmith for
- REPAIRS OF INVESTARM MUZZLE LOADING FIREARMS
MUST BE DONE ONLY BY A INVESTARM AUTHORIZED