Tabulate the distance of all points of impact above X'X' from X'X' as follows: 12.2 Divide this sum by one-half, 1⁄2, the total number of shots, which equals 12.2÷5=2.44= mean vertical deviation. Similarly, tabulate the distance of all points of impact on one side of the line Y'Y' from Y'Y', and divide the sum by 1⁄2 the total number of shots. The result is the mean horizontal deviation. The mean absolute deviation equals the square root of the sum of the squares of the mean vertical and mean horizontal deviations, and is expressed M. = √(M2 + M2 For. 41 The accuracy of the rifle is determined by the mean horizontal and mean vertical deviations. What C means to the ballistic properties of the projectile, the mean deviation means to the accuracy of the rifle. All shots fired, with few exceptions and these determined mathematically, should be counted in determining the deviations. To shoot a great number of shots at a target, then cut out the ten in closest proximity, and with these determine the deviation is a practice to be condemned. Generally, a careful inspection of any photographic illustrations of such groupings may be detected. However, to be fair to the rifle, to bring out its greatest capabilities these 10 shots or more should be fired under the best atmospheric conditions, using a machine rest. A reasonable time should be allowed between shots to maintain as nearly a uniform temperature of the barrel as it is possible. In fact the greatest care should be devoted to keeping the temperature of both rifle and ammunition as uniform as possible. Let it be understood that it is not necessary that the center of impact be the center of the target to determine the accuracy of the rifle. The accuracy of practice is measured by the distance of the center of impact from the center of the target, but the accuracy of fire, which determines the accuracy of the rifle, is the mean deviation from the center of impact. The accuracy of fire depends upon, The accuracy of the rifle, The uniformity of atmospheric conditions along the trajectory, etc. The selection of a cartridge properly loaded to meet the requirements of the individual rifle. The uniformity of ammunition, The uniformity of temperature of barrel and ammunition. CHAPTER XV. The Angle of Departure, Angle of Jump, the Graduating of Sight for Distance and Windage, Zeroing the Rifle, Cant, Holding Over Game, Etc., Etc. Problem 27. Given V (Velocity) (2700) X (range in feet) 500 yards or 1500 feet. f. (atmospheric factor) .1. To determine the angle of departure, a. Sin 2a AXC For. 42 in which a is the angle of departure, and the value of A is taken from Table A. X Z is at the head of column 1, Table A. For. 42a In Table A, Page 160, under 2700 f.s. velocity |