Weapon Training: Gunsights - N-3/9 USAAC/USAAF - Basics

A machine gun in an airplane is fired normally at an indicated air speed of more than 150 miles per hour at a target which usually is moving at a high speed. Conditions and time do not permit firing long bursts with the hope of spraying the target sufficiently to insure a vital hit.

Be ready to take immediate advantage of the few seconds available during which the target is exposed to deliver accurate, effective bursts.

Aircraft machine gun sights are designed to enable firing with accuracy under all normal combat conditions. To use the sights effectively, have a thorough knowledge of the proper use of the various types of sights.

Theory of Lead

To understand clearly the theory of lead it must be remembered that to obtain a hit, the bullet and the enemy airplane must arrive at the same point simultaneously. It takes a certain period of time for the bullet to travel the distance between the two airplanes. The center line of the bore of the gun, if extended, should not pass through the target, but should strike at some point along the apparent line of flight of the target at an appreciable amount in front of the enemy airplane.

The radius of the larger ring of a sight is such that the angle between the edge and center of the ring is, equal to a definite lead angle. In the case of ring and bead sights the gunner's eye must be a definite distance (usually 8 inches) from the ring. This angle may be expressed in terms of mils, or in terms of miles per hour of relative speed perpendicular to the line of sight. That is, a 150-mile-per-hour ring establishes the proper lead angle at the gunner's eye when the target is traveling at a relative speed of 150 miles per hour perpendicular to the line of sight. This is exactly true for one particular range only, usually 400 yards. However, if the relative speed perpendicular to the line of sight is constant, then the change in the lead angle is small as the range changes.

Reflex Sights

The basic principle of all optical sights is the projection of the image of a reticle of some form out to infinity so that the gunner can aim the gun by moving the sight until the reticle appears to be superimposed on the target. The reticle size is computed so as to be an aid in range estimation. With this sight it is not necessary for the gunner to hold his eye very steadily at a fixed distance from the sight while lining up a pair of reference points with the target. When the distance between the gunner's eye and the sight is too great, it is possible that part of the reticle will not be seen. This is purely a function of the size of the reticle and the physical diameter of the lens system used and does not affect the use of the sight so long as the reticle can be seen. When the complete reticle can be seen, it can be used to estimate lead or range.

NOTE: Check your sight lamp before take-off.

REFERENCE: Technical Order No. 11-35-1, dated September 18, 1942.