A method that was used in the days of make do and mend to tighten threads mainly because threading of parts could be a little hit and miss to say the least.
I have used this but never on a gun but I do not see any reason why it should not be successful.
Wrap some fine Copper Bronze or soft Iron wire in the nipple threads this will take up the lateral play in the thread and make things tight again.

The only lessons in my life I truly did learn from where the ones I paid for!