Along with what others have said, here is some worthless $.02 from me.
hard to say the original cause.
It could of been a "soft" metal problem.
But the drums aren't as hard as the barrel anyway.
The second time can be attributed to the above "soft" metal and the fact that after you tapped it out, a portion of the thread area is now thinner, making it weaker.
And as you now know someone apparently drilled and tapped the clean out screw weakening it further.
Hard to tell for sure from the pictures, but it does look rusted a bit in areas.
if it is this could come from.
Numerous wet swabbing and not thoroughly drying and letting it sit afterwards still damp.
The corrosive nature of Black powder and not being thoroughly cleaned afterwards and lubed.
A lot of people insist on cleaning with soap and hot water.
problem is soap acts like an adjuvant. Meaning it helps it penetrate better. Farmers use adjuvants in the field so fertilizers and such soak into the ground better and are absorbed by the plant better, also are used in medicines.
So in this case the solution more easily seeps into the threads. Weakening the metal from there.
So as you see several things could be working together causing the issue.
Suggestion, use the proper size tap for the drum threads, install a new drum. and a new nipple and try again.
Don't do a lot of wet swabbing.
If you insist on soap and water cleaning. Use a hair dryer or such to evaporate the moisture in the drum area. combination of heat and forced air, will dry it pretty well.
I have only used Rubbing Alcohol on my 50 CVA over 35 years old. No signs of rust or impending failure.
But glad to hear you did get it working again.
