I might be wrong but I think your rifle uses a snail breech.
If this is true, the hole thru it will need access to the powder charge so you must use a washer type seal.
Until you know how thick the washer needs to be you can't make a good decision about what the best thing to use would be.
If you have a precision depth guage like the one that is built into a vernier, dial or digital caliper, remove the breech plug and measure the distance between the end face of the threaded area and the face that will fit up against the rear of the barrel.
Then, measuer the depth from the rear of the barrel down to the shoulder at the bottom of the threads.
That shoulder is what the seal will press against when the breech plug is screwed in.
If the difference in measurement is only a few thousandths of an inch (.001-.020), some brass shim stock could be used. The shim should be 1 or 2 thousandths thicker than the gap.
If it is more than .020, a thin brass washer might be used.
For instance, if the difference in measurements was .026 you could take a .032 thick washer and sand it down to .027 thickness on a wet/dry black silicone sandpaper on a piece of window glass.
Then, if this is too much to ask, you could wrap a few turns of teflon pipe thread tape around the threads on the plug.
IMO, there is no way you should even consider using JB Weld on the threads.
If you do, it will be almost impossible to ever remove the plug without first heating the area to over 450°F.