With the big switch back in the 1960's from paper to plastic rust in the chambers became a common problem. The paper cases had left a protective coating of wax in the chambers. Bore cleaning & oiling had been sufficient but was no longer the case. An ordinary bore fitting brush/mop etc may not be large enough to sufficiently take care of the chambers. You may need to use a larger chamber specific brush to ensure they are adequately protected. I seriously doubt this is powder specific.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra