If this gun was as dirty as you say, I would either check the condition of the drum threads, or have a knowledgeable gunsmith do so. I was a contestant in a long gong shoot at a Rendezvous in Northern California, about thirty five years ago, when another contestant shooting a GRRW trade rifle blew the drum out of it. It wrecked the lock, and narrowly missed a spectator several yards away. This gun was nearly new, but the piece of drum thread left in the barrel indicated the thread had been cracked for some time. Maybe from over torquing when it was built. Better safe than sorry.
Hungry Horse