In general, fine threads are made to closer tolerances and are stronger
Unified national coarse. UNC is the most commonly used thread on general-purpose fasteners. Coarse threads are deeper than fine threads and are easier to assemble without cross threading. The manufacturing tolerances can be larger than for finer threads, allowing for higher plating tolerances. UNC threads are normally easier to remove when corroded, owing to their sloppy fit. However, a UNC fastener can be procured with a class 3 (tighter) fit if needed (classes to be covered later).
Unified national fine. UNF thread has a larger minor diameter than UNC thread, which gives UNF fasteners slightly higher load-carrying and better torque-locking capabilities than UNC fasteners of the same identical material and outside diameter. The fine threads have tighter manufacturing tolerances than UNC threads, and the smaller lead angle allows for finer tension adjustment. UNF threads are the most widely used threads in the aerospace industry.
There are more elaborate thread form for even greater strength, like the "J" form thread:
UNJC and UNJF threads. "J" threads are made in both external and internal forms. The external thread has a much larger root radius than the corresponding UNC, UNR, UNK, or UNF threads. This radius is mandatory and its inspection is required, whereas no root radius is required on UNC, UNF, or UNEF threads. Since the larger root radius increases the minor diameter, a UNJF or UNJC fastener has a larger net tensile area than a corresponding UNF or UNC fastener. This root radius also gives a smaller stress concentration factor in the threaded section. Therefore, high-strength (180 ksi or more) bolts usually have "J" threads.
These more sophisticated thread forms are more than what would be required for a breech plug application in the traditional barrel materials.
Of more importance than the thread coarseness is the class of fit. A sloppy fit will negate whatever strength there may be in any thread form or number. The threads need to be a very close fit to achieve full strength. Here is the definition of thread classes:
Thread classes are distinguished from each other by the amounts of tolerance and allowance. The designations run from IA to 3A and IB to 3B for external and internal threads, respectively. A class I is a looser fitting, general-purpose thread; a class 3 is the aerospace standard thread, and has a tighter tolerance. (The individual tolerances and sizes for the various classes are given in the SAE Handbook
In general, a fine thread with a Class 3 fit is the strongest. And, if you want to get crazy, there is a Class 5 category which is an interference fit that must be precisely made because the external thread is actually larger than the internal thread and, as the material yields during assembly, the threads are exactly the same size when fitted. Easy to screw this up and make the external threads too big to be assembled.
Bottom line, for a black powder firearm using typical barrel materials, using either fine or coarse threads will work and that point has been proven by many barrel makers over centuries. I just like to make them fit tight.
