Its smells kinda like if you had a little tobacco juice and put it out in the sun for a few years .
Like a spittoon that hasn't been emptied for months!!!!. My first Mini, from Classies, wreaked so strongly, of a really strong tobacco, that the package fouled up the air, in the car, driving home from town. When I opened it it almost knocked me out. Luckily it didn't have a mildew smell to it. I think I only used 99% alcohol, that did work. The last thing to hold the smell was the cap o-ring that I soaked for several days.
I didn't take the tube off but did fill the bottle with the alcohol, used a syringe to get the tube totally filled and then turned it upside down, in dish strainer, to soak for a few days. You have to have the o-ring, in cap, to keep it from leaking out. It shouldn't leak out if kept at same atmospheric pressure.
If I'm remembering correctly part of the firing pin is covered with some tubing that appears to be silicone. There may be some odor trapped on the inside of that tubing.
Some of the things that will take mildew odors out of fabrics might leave an undesirable scent that you don't want to be smelling when vaping, like a fragrance. Baking soda or borax,
not both mixed with water, comes to mind, also, for removing odors. Have you tried denture cleaner/soak?
I'll do a google search.
Alice