Thanks Daryl,
I'm very fond of Kroil but due to its scarcity around here I tend to use drops instead of ounces! Have been applying Kroil off and on since I brought the gun home, and also applying a few drops down-barrel with barrels then positioned for best effect. Have also done the tapping, tho not daily, and used some modest heat. Really need to build close-fitting nipple wrenches for the non-matching nipples. One square, the other typical rectangular base but worn.
A friend who works with old cars suggested a twist on ordinary heating which I will try: heat some appropriate object (old screwdriver in the case of screws) that fits the stubborn part well til it is red hot. Apply to the frozen part so the very localized heat works to break up the metallic adhesion between the threads and the hole. Part usually comes out he says.
I've certainly tried every other way of getting out old gun screws and nipples, and so far haven't broken any off. PATIENCE and TIME are the keys, and appropriate well-fitted removal tools. I know I'm "preachin' to the choir".
Aloha, Ka'imiloa