Column 10 is headed “Change of range for variation of + 10 feet per second initial velocity.” The sole use of the plus sign in the heading is to indicate the direction of the error in range resulting from a variation from standard initial velocity, an increase in initial velocity naturally causing an increase in range.
The two elements which affect initial velocity are gun erosion and variation in powder temperature. Of these, erosion causes a loss in initial velocity and a decrease in range. The Navy range tables are based on a standard powder temperature of 90° Fahrenheit. Since magazine ventilation is regulated to ensure that the temperature in magazines hardly ever exceeds this figure, it is obvious that an increase in initial velocity due to powder temperature seldom occurs. (It may occur if ammunition from ready lockers, located topside is used.) The variation in velocity due to temperature variation of one degree from 90° F. is stated in certain range tables and other Bureau of Ordnance publications, a reduction in powder temperature causing a loss in velocity. The loss of initial velocity per degree change of powder temperature will vary with the caliber and powder index. However, 2.foot-seconds per degree F. change of powder temperature is about average, and will be used in this text for simplicity.
The loss of velocity caused by erosion is dependent upon the amount of bore enlargement, as was discussed in chapter 5 of this text. The range table includes one graph which shows the amount of velocity loss for specified amounts of bore enlargement, which can be used following star gaging, and a second graph showing bore enlargement versus equivalent service rounds fired, for use in periods between star gaging.
It must be noted that the loss of initial velocity for bore enlargement is, in the case of the 5”/38 caliber gun, a loss from new gun I.V. of 2600 foot-seconds, 2500 foot-seconds being the average velocity during the life of the gun; therefore, in a relatively new gun, velocity change due to bore enlargement may have to be computed, in relation to the range table, as a gain.
Having obtained the velocity loss from powder temperature and erosion data, as described above, one may determine the resulting range error from column 10.
Example: Find the range error due to a loss of initial velocity of 126 foot-seconds from new-gun standard for the 5”/38 caliber gun, firing at a range 10,000 yards.
Solution: From column 10, at range 10,000 yards, the error in range for a loss of velocity equal to 10 foot-seconds is -48 yards. A loss of 126 foot-seconds from 2,600 foot-seconds represents a loss of 26 foot-seconds from the range-table initial velocity. The error due to a loss of 26 foot-seconds is 26/10 X (-) 48 = -125 yards, or 125 yards short.
Note: A shot falling short of the target is termed a short, while one falling beyond the target is an over. Shorts and overs are corrected by add and drop spots, respectively.