Generally you want to make sure that the major diameter of the thread is as close to the original external barrel dimeter as possible - to maintain as much wall thickness in the barrel as possible.
This is why the fatter the barrel, generally the large dia the muzzle thread will be because you want to maintain as much thickness as possible.
Basically muzzle threads are usually chosen because A. popularity of accessories in a given thread. B. remaining meat left on the barrel with chosen thread.
Its debatable what if any effect an undersized thread would have on a large dia barrel - The main effect id be concerned about is the potential for the thin parts of the barrel to heat faster than the thick parts. When steel gets hit it distorts. This could potentially lead to inconsistent distortion caused by heat where the end of the barrel changes (due to heat) at a different rate/degree to the rest of the barrel; potentially causing consistency issues at different barrel temps.
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