breech plug seized up on me.

Some drops of kroil should do the trick. That assumes you've already cleaned the whole thing with extremely hot water.

Instead of just a "grease"... I'm a fan of using an anti-seize product. On all my bike's fasteners... I like a dab of an assembly lube with moly/graphite to keep my wrenches working less strenuously.

What kind of powder did you shoot - I'm guessing 777 ???
If so, it's bad about sticking breechplugs.

I use Breech Plug Tape on my plug, along with a little bore butter on top of the tape before I thread it in and I haven't had a problem since. CVA makes this tape (it's pink) but you can also get the heavy duty (chemical resistant) teflon tape at any hardware store and it's the same thing and works great.

Also, when you install the plug - don't torque it down like a gorilla .
Screw it in until it hits bottom, then just a quick snugging is all that is required.

I'm wary of how tight female threads can strip the grease right out of the breech plug threads while it's being screwed in.
I would spread some of the grease on both the female and male threads, then insert the plug in and out several times to really distribute the grease.
Swab the bottom of the barrel to pick up any of the loose grease that gets pushed inside, and then double check the flash hole opening to make sure grease isn't blocking it.
Give the breech plug a partial turn 1/2 through an extended range session or after a certain number of shots.

Ya been there done that. if you have a vice. Take the barrel. Wrap it up in a towel. Then tighten down the vise really good. Spray with your penetrating oil. Then put that wrench they give you on it. Then take a long pipe and put it over the wrench for extra leverage then slowly lift up on the pipe. Any how i have had mine stick about 3 times on me now.

Now i know what to do. At the range. My son and i only fire around 5 shots. Then unscrew the breech plug and give it a cleaning. Put on some antisieze again then its usually good to go. Same time for accuracy after every shot the barrel gets a swabing.

breech plug

20 years ago, I went to a machinist supply place in Hyattsville MD and bought a pint of some stuff called Never Sieze. It is a silvery colored greasey stuff a little more fluid in consistency than grease. It is great stuff. Every nipple gets a dab after the gun is cleaned and when re-assembled. I put it on every breech plug, in case it must be pulled. Touch hole liners, etc.

I think it cost me $6.00 for the pint and it will last well past my funeral.

At the check-out at Advance Auto Zone, they have little packets of anti seize. Probably a tablespoon full in them. Handy to keep around.

+1
On the teflon tape. The Anti Seize does work well, but adds a bigger mess to clean up. IMHO