After trying just about everything. I hands down found the best stuff out there for breech plugs. It is made by Jet-Lube. It's called NIKAL. It's pure nickel for extreme service, anti-seize lubricant. It protects against galling, seizure and heat freeze which you can get from products like vasoline.
Temp range is -65 f to 2600 f.
I also use it on all my chokes for my waterfowl guns, etc. Anything threaded that I want to later come appart. Great stuff.