[rammed]

Well as far as your question on two triggers goes. It's a double set trigger. When you cock the hammer, you then pull the back trigger. This "sets" the front trigger and it's a "hair trigger. It's adjusted for a very light pull.

Your new pics shows a lot of corrosion around the nipple. I think that is what you are calling a percussion cap. A percussion cap fits on the nipple and makes the gun discharge.

I doubt if the previous owner cleaned the gun and the barrel will be to far gone to save. Powder residue is very hygroscopic and draws moister.

To take the gun apart, Cock the hammer and remove the screw that holds the lock on. The lock will not fall apart when you remove it. Everything is screwed together on the lock. Remove the screw from the tang. You are correct about it screwing into the trigger assembly. Remove the nail from the stock with a pin punch ( I called it a barrel key). The barrel will then lift out of the stock.

I would like to see a pic of the inside of the lock. I could tell you if it's an original or a Davis repro.

To take the barrel apart, Unscrew the drum from the barrel. The drum holds the nipple. Just take them out together. Then unscrew the tang form the back of the barrel. Use padded jaws in a vice to do that. Soak the barrel in some kind of anti-seize fluid first. Liquid wrench, Kroil, etc.

You will then be able to tel if the barrel is OK.

Doug