Adjustable Ranging Telescope
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Developed and patented by James M. Leatherwood, the Adjustable Ranging Telescope (ART) system combines a rangefinding scale inside the telescopic sight (scope) with an adjustable cam built into the scope's mount that raises or lowers the rear of the sight to compensate for the trajectory of the bullet. The cam is preset for a specific cartridge; i.e. the 7.62 mm NATO round. In use, the shooter first estimates the range utilizing the scale or scales visible through the scope, then adjusts the cam to that specific range. This then allows the shooter to place the scope's aiming point directly on the target without having to hold over or under to compensate for the bullet's rise and fall.
As a brief history; during the Vietnam War the U.S. Army found that they desperately needed snipers. They were losing troops to enemy snipers and had no capability of retaliating in kind. Their major problem was that the classic training of a sniper in range estimation, ballistics, compensation for weather or climate variables, and precision shooting is a lengthy process and they didn't have the time. About this time, Lieutenant Leatherwood, who already had the patents for his ART system, entered the Army and began working with U.S. Army Ordnance [1] (http://www.goordnance.apg.army.mil/Default.htm) specialists. The combination of the Leatherwood ART with M-14 National Match rifles to produce what became the M21 Sniper Rifle, allowed the Army to place competent snipers in the field with all line units.
Modern "civilian" versions of this system are sold by Leatherwood Optics [2] (http://www.leatherwoodoptics.com/) and a more detailed Manual of the scope and its mounting system are available there.