Is there any reason not to use a coarse threaded breech plug? 7\8 - 9 to be exact. It's going in a shotgun barrel, not intended for heavy rifle loads.
I'll assume you are threading a modern 12 guage cartridge barrel?
If so, the chamber diameter should be .813±.002" at the rear and .800" diameter at the front.
A internal 7/8-9-2B UNC thread has a major diameter of .875 min." and a minor diameter of .755-.778.
A internal 7/8-14-2B thread has a major diameter of .875 min. and a minor diameter of 798-.814.
Notice, the minor diameter of the fine 7/8-14 thread's minimum size is smaller than the 12 guage chamber but it's nominal minor diameter size is within the thread requirements.
The minor diameter of the course thread is considerably less than the 12 guage chamber size. If it is used in the 12 guage chamber it will result in a lack of thread surface being engaged with the male thread. This will weaken the thread strength.
On a more general note, the fine pitch threads have larger tensile stress areas and their larger minor diameter provides a larger sheer stress strength than the course pitch threads.
I advise using the fine pitch thread if it is possible.
If you are threading a un-breeched 12 guage barrel, the fine pitch threads higher strengths make it a better choice but either thread pitch should be capable of withstanding the breech pressures in a 12 guage barrel.