This information is direct from Lesson 40 of the Small Arms Training, Volume 1, Pamphlet No. 7, .303″ Machine Gun, Part I. – Mechanical Subjects, 1939. A full copy of this manual is available on the downloads page.
LESSON 40. – Slide Rule, M.G. Mark IV
On the front side are extracts from the Range Table, and a blank space for use as a writing tablet.
On the sides are measuring scales.
Attached to the rule is a cord, 24 inches long.
On the back of the rule is the following:–
i. Safety angle scale.
On the right centre of the rule is marked a range to target scale, and on the right side a range to troops scale.
ii. Degree scale.
On the right side of the right slide is a dgree scale and on the top of the rule is a line.
To use :– To measure angular widths – when the rule is held at the length of the cord from the eye, the amount of slide projecting at the top of the rule will cover on the ground the amount in degrees and minutes shown opposite the line.
iii. Wind scale.
On the reverse of the right slide are the minute allowances. On the right of the rule is a range scale. On each side of the slide on the rule are marked the wind directions as clock rays.
This scale is marked for a 10 m.p.h wind.
To use :– Place the arrow on the slide opposite the range to target. Opposite the appropriate wind direction lines read the range and / or line corrections in minutes.
iv. V.I. scale.
On the left slide is marked the H.E. scale and an arrow. On the rule is marked the V.I. scale and a degree scale.
To use :– With a known V.I. and H.E. the angle subtended can be found by setting the H.E. on the sliding scale opposite the V.I. on the fixed scale. The angle can be read opposite the arrow on the right of the slide. The V.I. or H.E. can be found in similar manner when the remaining factors are known.
v. Conversion of metres of yards.
On the left column of the rule, near the bottom, is an arrow marked “Metres to Yards.”
To convert metres to yards :–
Slide the left hand slide up until the figure 100 on the H.E. scale is opposite this arrow. Follow the H.E. scale up to the required number of metre. Read the figure opposite this on the V.I. scale. Multiply the figure on the V.I. scale by 10 to find the number of yards.
vi. Displacement scale.
On the reverse of the left slide is the conversion scale marked in degrees, and on the left centre of the rule is a degree scale.
To use :– Place the angle TOG opposite the oblique base (on the V.I. scale) and read the true base from the 90 degree index.
The range correction can also be obtained by adding 90 degrees to the angle TOG, when O is behing the gun position, or subtracting 90 degrees from the angle TOG, when O is in front of the gun position.
Place this angle opposite the oblique base on the V.I. scale; the resultant reading from the 90 degrees index will be the range correction.
Reverse the slide – place the range to target (H.E. scale on slide) against the true base (v.I. scale) and read off the angle by the arrow on the right. This is the angle by which the director setting differs from 180 degrees. The angle at which the director must be set is also shown against the arrow.
2. Practise squad.