17B5 RANGE TABLES COLUMN 11

Column 11 is headed “Change of range for variation of -1 pound in weight of projectile.” This change of range is due to two causes which are opposite in their effects. The first is the change in the initial velocity. A projectile heavier than standard will be expelled from the gun at lower than designed muzzle velocity, causing a decrease in range. However, increase in weight will cause an increase in value of the ballistic coefficient and, since retardation varies inversely as the value of the ballistic coefficient, this causes an increase in range.

Referring to the
range table in appendix C, part 2, it is seen that at short ranges, the effect on initial velocity is the predominant factor, while at long ranges, the effect on ballistic coefficient has the greater significance. At a range of 6,700 yards (in this table) the effects exactly cancel each other and the net error is zero.

In practice column 11 is not normally used aboard ship. It would be used if projectiles of other than standard weight were to be fired. Projectiles are required to be of designed weight within small tolerances, and the effects of these small variations are neglected, since it would obviously be impracticable to weight each projectile or to make a correction for each gun before firing each salvo. It is apparent, however, that this variation in projectile weight is one reason why all shots of a salvo, fired at the same time and under the same conditions, do not fall at the same point.

The standard weight of a 5"/38 projectile of 55.18 pounds a variation of 1 pound weight is the weight tolerance allowed.

The standard weight of a 2700 pound projectile a variation of 10 pounds is allowed.

Thus the 2700 pound projectile is much more accurate because of the smaller weight tolerance allowed.