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Old March 11, 2014, 12:41 PM   #9
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 8,927
Sight radius.

It was originally standardized in the USA and Great Britian on rifles as 30 inches a century ago or more. Adjustable rear sights mounted on the back end of rifle receivers were made with lead screws having 40 tpi. One third turn move the aperture .00833 inch which is exactly 1/3600th of a 30-inch sight radius; most barrels were 24 inches long so the front sight was about an inch back from their muzzles. There's 3600 inches in 100 yards. The rest of this should be easy for most folks to figure out.
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