Report: Analysis of the Action of a Bren Gun

TNA Catalogue Reference: DSIR 27/41

Context: Department of Scientific and Industrial Research: Road Research Laboratory Reports, Reports on War Researchers Authorised at the Road Research Laboratory

Scope and content: ID 1-30

Covering dates: 1941 May - 1944 Oct

Courtesy of Drew

 

Note No. ID/1/AGT.
May, 1941.

DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Road Research Laboratory

AN AUTOGRAPHIC ANALYSIS OF THE BREN GUN ACTION


SUMMARY

I. OBJECT OF TEST

II. ARRANGEMENT OF APPARATUS

III. RECORDS TAKEN

IV. DISCUSSION OF THE RECORDS

V. CONCLUSIONS FROM THE ABOVE

REPORT

APPENDIX

 

SUMMARY

A description is given of the application of an autographic recorder to the action of a Bren gun. Results of such tests are described, and a typical case analysed in detail.

The information yielded by this analysis has enabled a clear picture to be formed of the detailed movements of the gun parts, and particularly of the conditions of operation of the springs. In the light of this informations the question of fatigue in the gun springs and the effect of their stiffness has been considered.

The records have also indicated the factors affecting the firing rate of the gun; no large increase in rate appears practicable, but some increase can be obtained by changes in the springs.

 

I. OBJECT OF TEST

Information was required by the National Physical Laboratory on the impulses, velocities, and accelerations occurring on different parts of the Bren gun when firing, with particular reference to three springs, viz. the return spring, the piston-post spring, and the firing-pin spring. It was decided to measure the movement of the piston autographically, using a high speed mechanical recorders.

 

II. ARRANGEMENT OF APPARATUS
The gun was mounted on a specially built and very rigid stand, being held by the two anti-aircraft mounting lugs on the gun. The stand is shown in Fig. I, in which it is shown on a trestle for the purpose of the photograph: for the firing tests it was bolted firmly to a steel box filled with concrete, and weighing some two tons.

The recorder used was a motor-driven drum recorder, with a record speed of 100 in./sec. The speed was checked by a vibrating reed, and found to be 99.8 in. for some records and 100.4 in. for others; the difference between these speeds and the standard 100 in./sec. has been taken as negligible in comparison with other factors.

The movement of the piston was measured by a specially designed push-rod, which replaced the normal return-spring push-rod, and which protruded through a hole drilled in the return-spring rear cap. This push-rod carried a pen, which traced the autographic record on the drum. To eliminate any error due to the mass of the pen and rod, its weight was kept down to that of the normal push-rod which it replaced. For this reason it was made from aircraft quality steel tube, 1/4 in. outside diameter, turned down to 0.015 in. wall thickness for the greater part of its length. The push-rod, pen and recorder crosshead weighed 36.3 g. compared with 38.7 g of the normal rod.

Before use, the gun was cleaned and oiled with thin oil (equal parts "C44" and paraffin oil) as used for aircraft automatic guns.

 

III. RECORDS TAKEN

The following autographic records were taken

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There was no appreciable difference in gun behaviour between Records 7 and 8, showing that the gun was thoroughly warm when Record 7 was taken.

(Supplementary records were taken, for a special spring test, described below.)

(For the purpose of this note, the gas ports are numbered in order, 1 to 4, in order of increasing size.)

The above records showed that, with the largest port opening (No. 4), and consequently the most rapid rate of fire, the piston compressed the buffer spring through its full range and hit the stop with a mechanical impact. It seemed that this might cause a recoil shock to the operator, if the gun were fired at this rate from the shoulder, and tests were carried out on this point.

In these tests a number of short bursts were fired from the shoulder, lying prone, by an experienced machine-gunner and by two inexperienced riflemen. In no case was any serious shock experienced.

 

IV. DISCUSSION OF THE RECORDS

(a) General.
It was at once clear from the records that the gun may function at three main rates of fire, which may be described as "slow", "normal" and "fast".

Slow functioning occurs at gas port opening No. 2 on the first round; or the first two rounds, of a burst. (It would occur normally on all rounds at gas port No. 1, but no bursts were recorded at this opening). It is characterised by the piston not reaching the buffer spring, but having its motion reversed by the return-spring alone. The rate of fire is from 500 to 560 rounds per minute.

Normal firing occurs with gas port No. 3, or No. 2 with a hot gun. The piston reaches the buffer spring with a more or less considerable velocity, but does no fully compress this spring. The rate of fire is from 560 to 670 rounds per minute, and represents the normal rate of fire of the gun.

Fast (or "Crash") firing occurs with gas port No. 4, after the first round or two rounds of each burst. The piston compresses the buffer spring over its full range, giving a mechanical impact with the stop. The rate of fire is from 670 to 730 rounds per minute.

While the impact on the stop does not inconvenience the machine-gunner, it probably reduces the accuracy of the aim, and may reduce the working life of some of the gun parts.


(b) Detail Analysis.

(i) General.
It has not been possible, nor is it necessary, to carry out a detailed analysis of every record taken. Inspection shows, however, that the gun behaves with great regularity, the ammunition used - ordinary .303 ball ammunition of "machine-gun" quality - was very uniform. Such small variations as occur are probably due to small variations in ammunition.

One typical record (Round 1 of Record 8) had been analysed in detail. This was chosen as the rate of fire was about the most rapid that could be reasonably expected in service without "crashing" occurring. It was the first round of a 5-round burst, in which subsequent rounds "crashed". It was felt that this would give the movement analysis under the most severe accelerations likely to be met with normally, and therefore under typical but severe conditions.

The rate of fire was 658 per minute. The analysis showed that, except in one respect, the action is entirely "according to plan" - i.e., was in conformity with the dynamics estimated from the masses and springs involved. The exception occurred in the reversal of the piston by the buffer spring, which was recorded as more rapid than could be accounted for by the spring-stiffness as stated. The spring rate of the latter was therefore measured in the laboratory, and was found to be 26.6 lb. per inch and not 21.6 as stated. The recorded motion was in full accord with the measured spring rate.

(ii) Friction.
There is no evidence of exceptional friction at any point on the cycle, but moderate friction occurs throughout. Friction is specially noticeable during the unlocking of the breech-block, while the piston is in contact with the buffer spring, and during the final advance of the piston after locking the breech-block. A small but unexplained irregularity occurs in the velocity of piston shortly before it contacts the buffer spring. This may be due to a point of local friction: it occurs consistently on a number of records at the same point in the cycle.

(iii) The Numerical Analysis of the Cycle.
The story of the action can be most conveniently begun at the moment after the breech-block is locked, and before the firing pin is struck.

After locking before firing
The piston is then travelling forward with a velocity of some 110 inches per second, being urged on by the return spring, which is nearing its most extended positions. It is however being retarded by frictional forces, and its velocity is falling slightly.

The piston-post is at the rear end of its slide (in contact with Shoulder A - Fig. 2), its spring being at its full extension.

The breech block is stationary, against the breech of the gun, with its locking toe in the locking recess in the gun. The piston and post are sliding under it, the post approaching the heel of the firing pin, which is fully retracted and at rest under the action of its spring.

Impact with firing pin
The piston-post comes into contact with the heel of the firing pin, which is driven forward, compressing the firing pin spring, and firing the cap of the cartridge.

During this impact the piston is brought to rest, as the piston is hard agains the piston-post (Shoulder A), but a reverse velocity is immediately given to it, partly by mechanical impact, but chiefly by the sudden development of gas pressure in the cartridge.

(The gas pressure gives what is effectively an "impact" to the breech-block, which though locked, transmits this "impact" to the piston-post and so to the piston).

Records taken with a dummy cartridge show that the mechanical impact is more sudden, but gives a much lower reverse velocity, than that in a live round, showing that the reverse velocity is due largely to the development of gas pressure in the cartridge).

Firing
The reverse velocity is impressed on the piston-post, which thus remains hard in contact with the piston (Shoulder A).

The reverse velocity carries on with little change for about 1.2 milliseconds, during which time the piston-post is hard in contact with the piston, and travelling with a velocity of 80 inches per second.

Gas port opening
The bullet proceeds down the barrel, uncovering the gas port, and admitting high pressure gases to the cylinder. These reach the piston head 1.2 milliseconds after firing.

The gas pressure reaches its maximum value very rapidly (in something less that 0.3 milliseconds) and impresses an acceleration in the piston of about 600 g.

The piston-post spring is incapable of transmitting this acceleration to the piston-post, which thus comes away from its rear stop and takes up its front position. The time taken to reach its front position (Shoulder B ) is 1.8 milliseconds.

During this time the maximum gas pressure on the piston head is 1.2 tons per square inch. This gives the piston (against the action of the piston-post spring and the return spring) an acceleration of 600 g.

At about the same instant that the piston-post reaches its front position, the piston begins to open the escape ports in the cylinder, and the acceleration in the piston falls off rapidly but not instantly.

Unlocking
At the same instant the toe of the piston-post engages the heel of the locked breech-block and begins to unlock it. Under this impact, the piston-post which is just in contact with its front stop (B ), is driven hard against it. There is some evidence that it bounces slightly back from its stop, and is then driven hard against it again by the inertial of the breech-block.

The detailed analysis of the unlocking action is not possible from the record, but there is some evidence that it occurs in two stages, the first associated with the withdrawal of the breech-block from its locking recess, and the second with its impulse backwards. The first, which occurs some 4 milliseconds after firing, causes the piston velocity to drop from about 470 inches per second to about 250 inches per second, and the second operation, to about 200 inches per second, the minimum being reached about 7.5 milliseconds after firing.

After this time the piston velocity increases. The increase is due to the breech-block, which has just received an impulsive velocity, overtaking the decelerating piston, the piston-post which links them being necessarily at this point in its rear position (A). This occurs at about 8 milliseconds after firing, a maximum of forward piston velocity occurring at 10 milliseconds.

A rebound of the piston-post from its rear position, causes a second drop in piston velocity, which is recovered when the piston post, after its bounce, regains its rear position. The time taken for this bounce is about 3 milliseconds.

There are some indications of a second small bounce, but apart from this the piston-post remains in contact with its rear stop for the whole of the rest of the cycle.

During the subsequent backward movement of the piston, up to its impact with the buffer, the breech-block has a small amount of end-play on the piston, and its exact position relative thereto is not determinable. In a frictionless system, its inertia, coupled with deceleration of the piston under the action of the return spring would cause the breech-block to remain on its back stop. Friction may pull it off this stop, in practice, but as the end-play is small, the breech-block will in no case be far from its rear position.

At 18 milliseconds after firing, the piston having then travelled 4.36 inches from its front position, the empty cartridge case hits the ejector, but this has no discernible effect on the movements of other parts.

At 30 milliseconds from start, the piston having then travelled 6 inches, a rather curious short deceleration occurs, followed by a short acceleration. This occurs on many of the records. It appears to be due to a rather high local friction affecting the breech-block at this stage.

Buffer action
At 6.18 inches from start, the piston comes in contact with its buffer, its velocity at this point being about 100 inches per second.

The buffer is driven backward some 0.21 inches, and imposes an acceleration of 31 g., in addition to that of 13 g. due to the (nearly fully compressed) return spring. This reverses the motion of the piston and starts it on its forward stroke with a velocity of about 60 inches per second. Friction causes the return velocity to be considerably less than the approach velocity of the piston.

Feeding the next round
On the return stroke, the breech-block picks up the next round at 4.35 inches from the extreme forward position. The impact produces a hardly noticeable change in velocity of the piston, but the friction of withdrawing the round reduces the acceleration to 4 g. instead of the "frictionless" value of 9 g.

The time available for the round to emerge from the magazine read for engagement by the breech-block is 36 milliseconds.

Locking
When the piston has reached .75 inches from its forward position, the breech-block comes into contact with the breech of the gun, and is thereby brought to rest. The firing-pin, which must jerk forward slightly under its own inertia, does not do so sufficiently to risk firing the cartridge.

The piston carries on under the breech-block, its ramp running under the heel of the breech-block and lifting the latter into its locking recess on the gun.

The piston velocity falls from about 180 inches per second, to about 110 inches per second under the friction of the locked bolt.

The position and motion of all parts of the gun are now as at the beginning of the cycle, and the history repeats itself.

Gun record
Throughout the above analysis, no allowance has been made for the movement of the gun-barrel in recoiling against its buffer. This recoil is over before the piston reaches its rear position, and its only effect on the above analysis is that the opening of the gas port, and the unlocking of the breech-block occurs a little (less than 1 millisecond) after the time stated above.

 

V. CONCLUSIONS FROM THE ABOVE

(1) SPRING CONDITIONS
Seeing that the investigation is required in connexion with certain spring problems, the accelerations, etc. suffered by each spring may conveniently be extracted from the above history.

(a) Firing-pin spring.
This spring is fully extended throughout the whole cycle, except for about 0.6 milliseconds before firing, and 0.7 milliseconds after firing.

This momentary action begins with the front of the spring at rest, and its rear abutment receives a suddenly applied velocity of 110 inches per second, under which it is compressed 0.06 g.

A sudden acceleration is applied to the rear abutment, lasting for a very short time, but giving it a backward velocity of 80 inches per second, the acceleration being of the order of 1000 g.

The spring regains its full extension after which the force on the rear abutment is relieved by the piston-post moving backwards away from the heel of the firing-pin.

Throughout the rest of the cycle the spring and its abutment move together, and its accelerations are the same as those of the breech-block.

At one point - the moment of contact between the advancing breech-block and the breech - the firing-pin spring may be slightly compressed by the inertial of the pin itself. This will in any case be a very small and very momentary effect.


(b) Piston-post spring.
For the whole of the advance of the breech-block, and up to the moment of gas port opening, the piston-post spring is fully extended, the post being hard in contact with its rear stop. The most violent acceleration suffered is that due to firing, but at this time the spring is moving in one piece with its abutments, and any shock affects the spring solely by virtue of its own mass-acceleration. Though the acceleration is of the order of 2000 g., it is not likely that the mass-acceleration of the spring produces any serious stresses in it.

When the gas port opens, 1.2 seconds after firing, the front abutment receives an acceleration of 600 g., tending to close the spring, and the rear an acceleration of 160 g. tending to open the spring. This lasts for some 1.8 milliseconds at the end of which time the forward acceleration of the front abutment has fallen greatly, but nevertheless continues to be greater than that of the rear abutment, and the spring reaches is fully-stressed condition.

It has just reached this condition when the unlocking of the gun drives the rear abutment hard into contact with its stop, and keeps the spring in its fully-stressed condition.

After unlocking, the rear abutment moves away from its front position and regains its rear position, having been away from it for some 6.8 milliseconds.

A small bounce occurs, the rear abutment bouncing in its rear stop, and regaining its rear position after some 3 milliseconds.

No other incident of interest occurs involving the piston-post spring throughout the rest of the cycle.

(c ) Return spring.
This spring has its rear abutment fixed throughout. Its history is that of the piston.

Two critical points occur in its cycle:-

(i) At the moment of firing, when an acceleration of the order of 2000 g. is applied momentarily to the front abutment, followed by an acceleration of 600 g., the latter lasting an appreciable time. When these occur the spring is only very lightly stressed.

(ii) At the moment of reversal of movement at the rear end of the stroke. The spring is then fully compressed, and heavily stressed, though the acceleration of the abutment - 44 g. - is not in itself serious.


(2) THE REDUCTION OF SPRING FATIGUE
It is understood that the above three springs are liable to fail through fatigue, and it appears that the stresses involved (as deduced from the static spring compressions) trespass on the fatigue range of the steel. The dynamic conditions will increase these stresses to a small extent, but it is not thought that this is a serious factor.

The life of the existing springs shows, however, that the fatigue limit is not grossly exceeded, and it appears that a small reduction of stress would bring the conditions below that limit.

Consideration has been given to means for reducing these stresses and three possibilities present themselves.

(i) The use of springs of larger outside diameter. This is not easily possible, owing the the room available in the gun.

(ii) The use of compound springs. This would introduce complications which are undesirable, though it would relieve the stresses considerably.

(iii) The use of weaker springs. In this connexion it must be noted that a reduction in the diameter of the spring wire will reduce the stiffness of the spring and the stresses in the wire. The relation between these two is a little complicated, but it may be simplified with the statement that to reduce a stress by 5 per cent, the spring stiffness must be reduced by 18.5 per cent (if other things are kept the same). By making use of all the space available - i.e. increasing the coil diameter as the wire diameter is reduced, the same reduction in stress can be obtained with a somewhat less reduction in spring stiffness, actually 11 to 12 per cent according to the proportions of the spring.

In the case of the firing-pin spring, the function of the spring could be performed safely with a considerably weaker spring: a 20 per cent reduction would be amply safe.

In the case of the piston-post spring, the load comes on the piston-post at a moment when the post is in contact with its stop, and the spring is therefore out of action. There is thus no reason why a considerably weaker spring could not be used (see below).

In the case of the return spring, a weaker spring would throw more load on the buffer spring, and increase the tendency to "crashing" (i.e. impact on the buffer spring stop). The buffer spring would need to be made somewhat stiffer, or be given more travel. Either of these could be arranged.

If this were done, a weaker return spring would give a more rapid rate of fire, but a slightly increased delay between pressing the trigger and firing the first round.

The general conclusion is, therefore, that somewhat weaker springs could be used in these cases with advantage.

To test this, a very much weaker piston-post spring was fitted to the gun, having a spring load of 6 lb. as compared with the normal piston-post spring load of 60 lb. The behaviour of the gun was normal and records show no significant difference from those obtained with the normal spring.

Two cases of pierced caps occurred, in 40 rounds fired. This was due to bounce of the piston-post under the very weak spring used. It is, however, clear that the gun will function correctly with a very great reduction in piston-post spring stiffness and there would be no danger of pierced caps if the stiffness were reduced by a reasonable amount.


(3) THE RATE OF FIRING OF THE GUN
It is understood that it is desired to increase the rate of fire of the gun. Elementary dimensional analysis shows that the rate of fire is proportional to the square root of the spring forces, and inversely proportional to the masses involved, for dynamically similar systems. That is, to achieve any considerable increase in rate of fire would mean a very great increase in spring stiffness, or reduction in mass of the moving parts. Neither of these seem practicable.

A small increase in rate of fire could be achieved by weakening the return spring and stiffening the buffer spring. This appears desirable from the point of view of spring fatigue.

It would, however, give a small increase in the delay between pressing the trigger and firing the first round. This delay is normally 0.07 second. A 10 per cent reduction of return spring stiffness would increase this by about 5 per cent, viz. by 4 milliseconds. It does not appear that this is a serious objection.

It means, therefore, that no spectacular increase in the rate of fire can be hoped for, but a small increase could be achieved.

 

Fig. 1
Bren Gun arranged for Autographic Recording

 

Fig. 2
Diagrammatic sketch of working parts

 

Fig. 3
Typical Record showing Slow and Slow Nominal Firing

 

Fig. 4
Typical Record showing Rapid and "Crash" Firing

 

Fig. 5
Typical Record showing Normal Firing

 

REPORT ON AIR OPERATED BREN GUN

 

Appendix