EXPERIMENT IN INDIRECT FIRE

The following is an extract from an article by First Lieutenant A. E. Phillips of the 10th Cavalry, from the Journal of the United States Cavalry Association for July 1909:

“To determine how many, if any, of the bullets from the machine gun would strike troops in front of an assumed ‘hill’ over which the gun was to fire, canvas frames were used to represent such objects, the targets being concealed from view.

“The target consisted of a strip of target cloth 6 ft. high and 15 yards wide, along the bottom edge of which is pasted a row of kneeling figures with an interval of a yard from centre to centre. Across the target and parallel to its top edge was drawn a narrow black line tangent to the tops of the heads of the figures. Value of hits on any figure = 5; value of hits on the cloth below the line = 3; value of hits on the cloth above the line = 1. Canvas frame, 8 ft. high, placed 200 yards in front of the gun. Rapid fire:

First Experiment

Range 800 yards


No. of shots.	No. of Hits.				Remarks.

	Figs.	5’s.	3’s.	Total.

30	5	10	12	22	Line of sight was 5 ft. below top of obstruction. All shots over.
30	8	10	9	19

Second Experiment

Range 1,000 yards


No. of shots.	No. of Hits.				Remarks.

	Figs.	5’s.	3’s.	Total per cent. of figs. hit.

30	9	11	4	60	Line of sight was 3 ft. below top of obstruction. All shots over.
30	12	17	7	80

Third Experiment

Range 1,200 yards


No. of shots.	No. of Hits.				Remarks.

	Figs.	5’s.	3’s.	Total per cent. of figs. hit.

30	2	2	5	13	Line of sight was 3 ft. below top of obstruction. All shots over.
30	4	5	13	27
30	8	11	9	53

“It will be noticed no 1’s were made. Assuming the height above the ground of the average mounted soldier as 8 ft., had a troop of cavalry mounted been 200 yards in front of the machine guns in the third experiment, the line of sight would have struck about the backs of their horses, and all bullets would have gone over the riders with at least 4 feet to spare, as proved by the experiment.... The troop mounted could have moved forward to within 100 yards of the target and would not have been struck by the bullets.”

The methods of carrying out indirect covering fire will vary according to the nature of the position selected and the way the guns are to be laid. This is one of the few occasions when a battery or two may have all their guns in line close together. If the slope is a steep one, they may be pushed up close to the crest, but on a gentle slope they must be placed sufficiently far back to avoid the forward effect of shrapnel bursting on the crest line.

The battery commander will select the position for the guns and will then give each gun or section a section of front to fire on. The method of marking off this section is as follows: two iron rods about 3 ft. long for each gun are painted white, and are then “laid off” from the gun to the target, in an exact line, the first being on the reverse slope a few yards short of the crest; the next on or just behind the crest line. By aligning the gun on the rods, fire is brought on the centre of the target; should traversing fire be required, the limits may be marked by similar sticks on either side.

There are two methods of laying the gun. (1) By observation of fire. (2) By quadrant elevation. The first method has been already described, and the second must be done by using the following table in conjunction with an instrument for elevating the gun at the required angle; but it must be remembered that each gun will require adjusting to its own angle of elevation to suit its known error at each range. Thus a gun which is known to require sighting at 900 yards when firing at 1,000 should receive 1° 10½ and not 1° 25½. When proper instruments are not obtainable, very fair results may be obtained by using an ordinary clinometer. The necessary allowance for the height the gun is above the target must be made, or, in other words, the angle of sight as shown by the clinometer must be deducted from the quadrant elevation given on the gun.

Diagram IV
TO SHOW METHOD OF LAYING BY AIMING POSTS

aa. Aiming Posts (about 15^x apart and some distance in front of gun.)

ttt. Targets.

TABLE OF ELEVATION FOR MAXIM ·303 USING AMMUNITION GIVING 2,000 F.S.


Range.		Angles of elevation on the Maxim gun.

200	yards	0	4·0	0 1
300	”	0	9·5	”
400	”	0	17·0	”
500	”	0	25·5	”
600	”	0	35·5	”
700	”	0	45·5	”
800	”	0	57·5	”
900	”	1	10·5	”
1,000	”	1	25·5	”
1,100	”	1	41·5	”
1,200	”	1	57·5	”
1,300	”	2	16·5	”
1,400	”	2	37·5	”
1,500	”	2	59·5	”
1,600	”	3	22·5	”
1,700	”	3	47·5	”
1,800	”	4	14·5	”
1,900	”	4	43·5	”
2,000	”	5	14·5	”
2,100	”	5	42·0	”
2,200	”	6	22·0	”
2,300	”	6	59·0	”
2,400	”	7	40·0	”
2,500	”	8	25·0	”
2,600	”	9	16·0	”
2,700	”	10	18·0	”
2,800	”	11	18·0	”

What is required is the angle of quadrant elevation to be put on the gun in order that the shots may strike the target at a known range. From range table obtain angle of tangent elevation; now if the target is below the guns we must subtract the angle of sight (m), if the target is above the guns we must add the angle of sight (m) to obtain correct angle of quadrant elevation.

To obtain angle of sight m

(1) When target is visible from the immediate vicinity of guns the angle may be measured by pocket clinometer or other mechanical means; or, if heights can be obtained from a contoured map, from the formula:

m = h × 1146 R

where h = difference in feet of height between guns and target; R = range in yards.

(2) When target is not visible from immediate vicinity of guns, m may be found from the following formula:

Diagram V

Where a₁ is the angle of sight from O to G; a₂ is the angle of sight from O to T.

Angles of elevation are read plus, angles of depression minus; T being the target, G the gun, and O the observer.

Before firing it is necessary to ascertain that the trajectory will clear the intervening crest—i.e. see that the angle of slope to the top of crest is less than the angle of Q.E. at which the gun is likely to be fired. If there is an obstacle some distance in front of the guns, to ascertain if the trajectory will clear it make a liberal estimate of the range to the obstacle and ascertain if the quadrant elevation to be fired at is greater than that which would be required to hit the obstacle.

If it is possible to place a mark to aim at near the crest with a given elevation which will ensure the fire striking the target, this will much simplify indirect fire, but the combination of circumstances required to enable this to be done will not often be found.

When all is ready to open fire the battery commander will take up a position from which to direct it, and will give the orders to open and cease fire and name the guns or sections to fire. The section commanders will watch the fire effect of their guns and give the orders necessary for correcting elevation or direction. The battery captain will assist the commanding officer by noting the fire effect.

It will be seen from the foregoing that considerable practice is required to enable indirect fire to be efficiently carried out; but the results obtained in covering the advance of infantry and thus enabling them to push up quickly to close range, without the delay and exhaustion caused by a prolonged fire fight, will more than repay the time spent in perfecting machine-gun detachments in this duty.