Cross-fire is where the projectiles from guns in different positions cross one another at a particular point of ground. (See [Fig. 105] for these cases.)
Fig. 105.
PROBABILITY OF FIRE.
Absolute certainty of hitting the same spot at each round is impossible of attainment; and accuracy of fire is therefore a comparative term.
The Probability of Fire measures the chance of hitting a given target. It is determined for any gun by firing a certain number of shot at a given range, measuring carefully the ranges and dividing the sum by the number of shots, which gives the mean range.
Subtract each range from this mean range thus obtained, and the results obtained will be the errors in range for each shot. Add these errors together and divide their sum by the number of shots, and this will give the mean error in range. Multiply this mean error by 1.69, and the product will be the depth in the direction of the range of a belt or zone which will probably contain one half the whole number of shots fired.
In the same way the width of the probable zone, laterally, may be obtained, and also the height of the probable zone vertically. The origin of reference for the points of impact of the shots in the last two cases is generally taken at the lower left-hand corner of the target.
The intersection of the first two zones will give a rectangle which will contain 25 per cent of all the shots.