I restored a repro 1863 (Zoli) Zouave with the same problem. First, let me preface this with the condition of the gun: it looked as if it had been shot every day in the 1970s, the put in the back of a leaky pickup truck without *ever* being cleaned.
Pulled the barrel first to protect the rest of the gun. I used an oversize nipple wrench (hex shank type-intended for inlines, I bought because it was expendable) and used a cordless drill with an impact-like function. I gradually increased the setting while soaking in CLP, and it *did* crack loose but would only turn about 1/8 revolution.
I really soaked it with the oil and gently rocked it back & forth by hand. About 4 days of oiling and rocking back & forth it unscrewed from the breech without damage to the nipple threads or the breech.
When it was apparent that the barrel was serviceable, I restored the rest of the gun. It turned out fairly well, too. I don't have any experience with CW era muzzleloading rifles, so I was surprised at the weight - it's much heavier than it looks.
Here are a couple of before & after photos. In the right photo, the stock has been cleaned up but not the metal. It shows the nipple replaced, and most of the barrel rust cleaned up.
I treat all the nipple threads of my percussion guns with automotive anti-sieze, and loosen & retighten once a year or so. That seems to prevent stuck nipples.
I chose to avoid heat, since it requires heating the stuck part past temper. Like walruskid1, I was an auto mechanic and also used the torch trick. It was handly for taking exhaust systems apart without ruining parts, but required skill with the heat. I understand the steel used in black powder guns isn't the best to begin with, and heating it beyond temper can't help.
I hope this helps somebody out there trying to save a percussion muzzleloader.
