Carbon Outlaw,
  The .180 are you are measuring is probably close to right for a 3/16 screw. (today's #10 screw is more correctly .189) The major diameter you can measure with a mike or calipers is always smaller than the nominal screw size.
Back in the day when they were using the rounded crests and valleys on those screws. The taps and dies were made from tool steel, very much inferior to todays high speed steels. Sharper peaks on the tap would have been more prone to breaking off and sharpening with a hone would have been more difficult.
The plus side of a screw with the large radii in the threads is they will be less likely to fracture, the down side is there was less bearing surface so in use they would wear faster.
The Unified system of screws was instituted in 1949 and used here till present. There was an American National system the came after the Krags and Springfields that predated the Unified system, my reference book did not give any dates for it.
Changes and improvements were coming fast and furious when these guns were produced.
  Making a replacement screw for any of these old timers is relatively easy if one has a lathe,  bench grinder, and a good drawing. I think the company that makes the scope makes for Krags also stocks screws.