Here is my experience regarding this issue. I took a gun to a shop that produces taps and sells numerous taps from other producers. They must have dozens and hundreds of different sizes and shapes, types and thread counts. They concluded it is 32 TPI, Whitworth, as I told him I understood it to be.
He used a "starter" tap and gently placed it into the threads in the gun; worked it into what he could feel as the starting point and, just with his fingers, made a quarter of a turn 3 or 4 times and gradually got it taking hold. When it got too tight for finger turning, after visually checking it numerous times, as he worked, he put a "T" wrench on it and worked it through. He only visually checked the screw and put it into the hole and once again gently turned it into place. It worked slick as anything. That was on a 1915 Arty. We repeated it on a 1900 AE with the same results. I bought the "starter" tap for $15 and brought it home to become a new addition to my tool box. Nothing was done to the screw. Apparently from the shape and bluntness of the screw, it is most often that the threads in the hole get damaged - not the threads on the screw.
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