It will be seen that the 75 per cent., or effective zone, is deepest at 500 yards, and gradually decreases as the range increases up to 2,000 yards; beyond this distance it increases again in about the same ratio up to 3,000 yards.
The following formula will give the effective zone (75 per cent. of shots) approximately for all ranges up to 1,500 yards inclusive: 50,000/Range + 20. Example for 1,000 yards: 50,000/1,000 + 20 = 70 yards, which is the depth of the area swept by 75 per cent. of shots, or the "effective" beaten zone. For ranges beyond 1,500 yards this formula is useless, and after 2,000 the beaten zone increases in depth, while the angle of descent of the bullets becomes so steep that the "dangerous space" is reduced to a minimum; and consequently the zone beaten by 75 per cent. of shots is no longer the "effective zone," and it will be necessary to get the target within the zone beaten by the nucleus, or 50 per cent. of the shots. This zone at 2,500 yards range is about 50 yards in depth, so an error in estimating the range of more than 25 yards over or under the correct distance will render the fire "ineffective." Even at 1,500 yards the "effective zone" (75 per cent.) is but 60 yards deep, which only allows an error of 30 yards over or under the correct range—a very small margin, even when using a range-finding instrument, but without an instrument it is obviously impossible to
