We can all agree that prevention, in the form of proper maintenance, is the best way to avoid these rusted in nipples and screws. However, like the original poster, you sometimes get ahold of a gun that was abused or neglected by someone else. That's what this thread is dealing with, and it's a good discussion to have. A lot of you fellows have offered some pretty good tips!
I've been cleaning and conserving an old M1863 Springfield that had been made into a half-stocked shotgun some time after the war. Not particularly valuable on the antique market, but still a part of our heritage.
I made sure it was not loaded.
It was pretty rusty, and the nipple was frozen solid. I removed the barrel and stood it up in a bean can full of plain old kerosene. I also poured some kerosene down the bore. After about six days, I took it out and tried a regular Treso nipple wrench on it but the nipple didn't turn. I then heated the bolster with a propane torch until it was too hot to touch but didn't change color, and tried again. No joy. I puttered around on something else until it had cooled down to just warm, then heated it again as before. This time, the rusty old nipple eased right out.
Some of the internal lock screws were also resistant to removal. I soaked the entire lock in kerosene for five or six days, and all of the screws came right out. I would like to say that jobs like this can remind you what a good investment a decent set of gunsmith's screwdrivers can be.
Anyway, a kerosene soak, carefully applied heat if needed, and properly fitted wrenches and screwdrivers frequently contribute to success in this sort of endeavour. I'll try to remember some of these other techniques and potions in case my own should fail me.
Best regards,
Notchy Bob