Trajectory, as measured in inches, is the rise of the bullet above a straight line between the muzzle and center of target, or point of aim, and is usually measured mid-way of the range. The term "rise of the bullet" is a technical expression, for in reality the bullet never rises above a straight line in extension of the bore of the barrel. The so called rise of the bullet has reference to the " sighting line," the line of vision from the eye across or through the sights and to the target.
Sights are not so set on a rifle as to make the sighting line parallel the bore, but the rear sight is affixed at a higher elevation than the front which causes the path of the bullet's flight to cut the line of sight, or as we say rise above it. If the line of sight really paralleled the line of bore, the bullet would be found to drop lower and lower the longer the range, and never rise above.
As it is, the bullet starts such distance below the line of sight as the front sight is above center of bore; then for some distance the ball travels beneath the line of sight until its path and the "line” converge when it "rises" above. From here on the ball continues to get farther and farther from the line of sight until it reaches a distance about six-tenths of the range, and then the two lines begin closing up until they meet at the center of the target.
The only true point-blank of a rifle is where the path of the bullet's flight first cuts the line of sight, and this is only a very few yards from the muzzle, depending upon the elevation of the rear sight and the height of front sight above center of bore. The second point-blank, where the bullet again cuts the line of sight, is a variable distance, though we can fix an arbitrary point-blank by saying that a rifle shoots pointblank up to the distance the bullet rises but a slight amount above the line, call it two inches. In practical big game shooting a bullet whose path never leaves the line of sight more than the two inches can be said to be shooting pointblank for the distance.
Askins, Charles. Rifles and Rifle Shooting. New York: Outing, 1912. Print.
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