The 25-pdr Field Gun 1939 - 1972: Part One - development and design

Introduction
Early Development
The 25-pdr Field Gun
Ammunition
Organisation
Tractors
Ammunition Trailers
Bibliography and Further Reading

Introduction

The last 25-pdr Field Gun to be used in action was manned by two SAS troopers in the Omani port of Mirbat on 19 July 1972 under the command of Captain Mike Kealy. The gun was used to support the SAS and members of the Omani Armed Forces who fought a running battle with Communist insurgents (called the Adoo) for several hours before an SAS relief column arrived. The gun in question survives to this day and was actually built in 1943. That such a weapon was still in action twenty-nine years after it was made is a testament to the design and durability of the 25-pdr as few modern weapons have had such a long service life, as modern weapon systems tend to be replaced on a regular basis when new ones are developed. For veterans of the Western Desert and most of those who served in the Royal Regiment of Artillery in World War Two (including one of the author's grandfathers), the 25-pdr became an icon, the most versatile British artillery piece of the war.

Early Development

The story of the 25-pdr is actually two stories, the first being related to the 25-pdr Mark I (also known as the 18 / 25-pdr) up to the point the British Army were evacuated from Dunkirk and the second relates to what was to become the most famous British artillery piece of World War Two, that of the Mark II. The weapon itself began as a requirement for the British Army in 1919, just after the end of World War One. At that point, the War Office had begun considering the need for future equipment, and once a requirement had been established looked at the various designs proposed by the various manufacturers. It would prove to be the designers at Vickers Armstrong that would become central to the gun's design and manufacture. The most important aspect to consider when looking at the 25-pdr was that it was designed as a gun / howitzer. It would combine the characteristics of the 18-pdr field gun (limited elevation but high muzzle velocity) with the 4.5in howitzer (high trajectory) from the First World War.

Artillery design is a trade-off, with a shell usually being designed first, to be a certain weight and to be fired a certain range, and this then affects the design of the weapon firing it (especially the barrel). In the case of the 25-pdr, both a stopgap version and a completely new design were conceived almost at the same time. This was complicated by the low priority placed on the development of new weapon systems after the horrors of the First World War. The Royal Artillery Committee, in conjunction with the Director, Royal Artillery, headed the process. In 1924, the Committee was looking at the various options and a range of some 15,000 yards was considered appropriate. Various calibres and weights of projectile were considered, most notably 3.9in and 4.1in projectiles, the thinking being that two guns would be needed. The former would be fired from a quick-firing gun and the latter a breach-loading howitzer that would be able to reach 13,000 yards with a thirty-three pound projectile.

The process languished for several years until 1933 when a requirement was drawn up for a 3.7in gun / howitzer to fire a twenty to twenty-five pound projectile to replace the two previous designs. In 1934 a General Staff Specification was drawn up and models ordered for testing by the Director, Royal Artillery, Major General H A Lewis, including the prototype of the 25-pdr gun. Due to budgetary constraints at the time, the authorities looked at what was still around at the time - there being many 18-pdr guns in store - and looked to see if the old equipment could be modified to accept the new 25-pdr projectile. By 1935, a decision had been reached to refit the old 18-pdr jackets with 25-pdr ones and adapt the gun carriages with the fitting of pneumatic road wheels so that they could be towed by a vehicle. This rebored gun was to be fitted with the larger bore rifled liner and became the first of the 25-pdr series, the Ordnance QF 25-pdr Mk I. The relining of the barrel was possible because the 18-pdr barrel was made up of a number of parts:

The rifled bore, known as the 'A' tube could be removed and replaced by a larger bore liner of about 87.5mm calibre. This new tube was what is known as autofrettaged, that is, subjected to internal hydraulic pressure so that the metal on the inner surface of the bore is stretched beyond its elastic limit, while the metal towards the outside of the tube retains its elasticity. This leaves the barrel in a state of compression and is therefore much stronger. Such a technique is still used today. Despite this, the old 18-pdr carriages hampered performance and only 11,800 yards could be achieved in firing tests. It was obvious that further development of the gun carriage was required if it was to cope with the extra stress that the additional range would generate. This would be a problem as the 18-pdr had a pole trail, i.e. a single pole that attached to the limber, originating from directly under the breech. Such a configuration naturally had an impact on the gun's characteristics but later versions of the 18-pdr had already started to move towards the design that was eventually be used on the standard weapon. These were the split trail (the Mk VP) and one that had a box trail (the Mk III / IVP). As it was, around 1,000 of the Mk I design were converted and issued.

Both Vickers and the Royal Carriage Department (RCD) at Woolwich produced a split trail design, but the gunners, who would have to use it in action, were pretty unenthusiastic about either. The Vickers split trail was considered in 1937 but the box trail actually confers a number of advantages in the field, including the fact that the muzzle can be elevated to a higher angle as the breech can pass between the sides of the box trail. In actual fact, one can look at the 4.1in BL gun design (1931) to see the design roots of the Mk I 25-pdr gun carriage, which itself originated from a 1922 box trail design completed for the Spanish Government, also used for the Model 1922 105mm gun that became the basis for the BL gun and similar to the design on the 18-pdr Mk IV. Vickers apparently produced the gun for trials on Salisbury Plain and its clearly shows the humped box trail and firing platform. This carriage was fitted with the Mk II barrel (at the insistence of the gunners) and was the beginning of the 25-pdr as we know it today.

The Mk II was a very different weapon to the Mk I (also known as the 18 / 25-pdr) but was in development at about the same time the 18-pdrs were being converted to the new ammunition. The gunners were keen to see a weapon that would marry the advantages of being able to traverse 360 degrees with a box trail and so the final design started to take shape. The Royal Artillery Committee had specified the need for a circular platform in a memorandum dated 3 March 1926. Such a feature was found on the 4.1in BL gun, but an extemporised version had also been invented by Hogg and Paul and used in 1918, being based on a spare wheel but essentially performed the same function as the later all-steel version. The platform was a steel disc that had projections around its edge and was slung underneath the trail. If the gun went into action, the platform could be lowered and then pulled onto the platform by the tractor. Once there, one man with a handspike could rotate it in any direction, a feature that would become invaluable in the antitank role at a later date. Much of the production work was carried out at the Vickers plant in Newcastle (some 12,253 being built there) along with the relining of the 25-pdr Mk I barrel, although its plants in Sheffield and Scotland also contributed and the gun was eventually manufactured in both Canada and Australia during World War II.

The 25-pdr Field Gun

There were in fact many different incarnations of the 25-pdr with a variety of marks used in service. The Mk I was a very different beast to the Mk II and there were three different versions of barrel, with two main versions of the carriage for the Mk I barrel, three versions of the carriage used for the Mk II barrel, and a whole host of adaptations for the carriage in general.

As mentioned previously, the 25-pdr Mk I was not designed from scratch but a development of the old 18-pdr from World War I, with the barrel being a conversion of the Mk IV 18-pdr barrel to the new ammunition. While not a bad gun, it lacked the design features of the Mk II that would make that gun such a classic. The barrel consisted of an autofrettaged loose liner, jacket, breech ring, oil reservoir and screwed collar. At the end was a single-action Asbury breech (after its designer) that utilised a stepped, interrupted screw (known as the Welin pattern) to ensure the breech could be operated in one smooth movement. Brass cartridges ensured the breech was gas-tight as the gun was fired.

The carriages used with the Mk I barrel were known as the Mk IIITP, Mk IVP and Mk VP, the 'P' being used to denote the use of pneumatic tyres on the carriage. The first two were very similar; being box-trail (consisting of a crossbar, suspension frame, braking gear, cradle clamp, shield and traversing gear) in design while that latter was of a split-trail design. The gun could be transported at high speed due to the suspension frame insulating the trail and cradle from the shock, while the traversing gear allowed them to be moved 4.5 degrees to the right or left and could be worked from either side of the gun. The buffer recuperator worked the same way it did on the later Mk II with a buffer absorbing the recoil and a hydropneumatic recuperator returning the gun to its original position. The Mk VP could fire with an elevation of 15 degrees with the legs closed, or 37.5 degrees with them open, had a maximum depression of 5 degrees and was fired from the left-hand side. The carriage also allowed the gun to move left or right by 25 degrees, a big improvement over other marks. The axletree, trail legs and traverse were altered from the others marks by having the trail legs connected to the axletree and crossbar, with the axletree having two stub axles fitted and each trail leg having a metal ball bracket at the front. Two stop brackets that connected by a hook to two eyes on the stub axle limited the movement. The gunners sat on a seat mounted on the top of each leg. The carriage body rested on a bracket connected to the axletree and was made of manganese bronze and steel. The entire upper assembly (shield, gun and carriage body) traversed on the axletree and could be folded down whilst travelling.

25-pdr Field Gun, Mount Etna 1943
25-pdr Field Gun, Mount Etna 1943

The Mk II had a carriage that was designed to give 40 degrees of elevation and 5 degrees depression along with a traverse of 8 degrees. The barrel was mounted on a hydraulic buffer with a hydropneumatic recuperator that all worked within a cradle box that moved with the gun box after firing. The breech was a vertical sliding block with a mechanical firing lock. The Mk II had a variable charge system whereby propellant charges were loaded by inserting separate bags into the metal cartridge case and a primer inserted into the base. A shell was placed in the breech first, followed by the cartridge. The Mk II had two firing platforms associated with it, the No. 9 (for the Mk I carriage that was the larger of the two) and the No. 22 (for the Mk II and III carriages that were narrower). Both platforms had toe plates that dug into the ground and the gun could be towed onto the platform in seconds where it had a 360 degree movement. When the gun was on the platform, the trail spade was normally covered up in order to prevent it from digging into the ground and hinder the gun being traversed quickly. The Probert scale (named after an artillery officer who invented it) acted like a mechanical computer and expressed tangent elevation (the difference between the line of sight and elevation) as a range between 0 and 45 degrees engraved on the base of the cone. Range scales measured in hundreds of yards were also engraved on the cone and a muzzle velocity reader arm could also be read off against it. The gunners used a dial sight as the main indirect fire instrument that could be revolved between 0 and 360 degrees so that prominent features in the landscape could be viewed. The bearing between an aiming point and a target could be calculated and the gun offset from the aiming point by the same degree measurement called in by the observer. The were various versions of dial sights used, the most numerous being No. 7A, No. 7C and No. 9 and for direct aiming the No. 29 or No. 41 sighting telescope was used. A pair of aiming posts was used if there was a lack of prominent features.

There were several versions of the gun barrel. The original was the Mk I with a loose liner. The Mk II was the standard design and was also manufactured in Canada being known as the C Mk II and used at the same time during the early part of the war, being differentiated by the linear silhouette of the barrel showing a larger muzzle area. The Mk III was introduced in 1944 and had a modification to the chamber (the shot seating) to stop a round falling back if the barrel was elevated high enough. The Canadian version was known as C Mk III. The Mk IV saw a modification to the shape of the breech ring that was an attempt to stop it cracking when the gun was fired and the Mk VI was introduced in 1964 having a breech ring made out of better quality steel. The breech was operated by a lever on the right side of the gun, while the gun was fired by a lever operated through a mechanical linkage on the left-hand side. The gun barrel was mounted on a buffer / recuperator mechanism with the whole thing being contained within a cylinder block. The gun was mounted utilising two gun straps with the breech end being fitted with two thrust collars, themselves being connected to two cotters as part of the recoil system. The entire block recoiled with the gun inside the cradle and the pistons were fixed to the cradle front cap. The buffer dampened the effect of recoil and controlled the later stages of run-out while the recuperator helped with the recoil, controlled the run-out process and returned the gun to its normal position.

The carriages that were used with the Mk II and later barrels were the Mk I, Mk II and Mk III. The Mk I was a humped box trail design that had a firing platform and a front shield. The Mk II was a narrower design, for use in the jungle but the main design used was the Mk III that was similar to the Mk I but incorporated the smaller No. 22 firing platform. The trail sections on the rear quarter were jointed, so that when brackets on the gun trail were repositioned, the jointing mechanism could work, enabling the gun to be elevated up to 55 degrees (rather than having to dig a gun pit).

Ammunition

British gunner fusing a 25-pounder shell
British gunner fusing a 25-pounder shell
The 25-pdr was provided with quite a varied selection of ammunition with at least four main types - high explosive, smoke, armour piercing and carrier rounds (for propaganda leaflets). The projectile and the cartridge were loaded separately - the charge being made up of bags of propellant that were placed inside a brass cartridge case (with a primer on the base), the number dependant on what ballistic characteristics you wanted to achieve. A 'normal' charge consisted of three coloured bags of propellant, but there was the option of adding an extra charge (called a 'supercharge'). The three charges together (red, white and blue) were meant to give the projectile a range of 10,790m (the 'red' charge having a range of 3,566m, while 'red' and 'blue' had a range of 7,132m), while adding a supercharge took it up to 12,253m. Despite having a modular charge system, the 25-pdr could still put up a decent rate of fire, as one source in the 4th Field Regiment, Royal Canadian Artillery had timed a gun firing 17 rounds per minute - an extremely impressive rate of fire, but probably not sustainable for long under combat conditions. The Mk I gun had high-explosive, smoke BE (base ejection) and armour-piercing shot. In 1940, these were the HE Mk 1D, the Mk 1D BE smoke shell and the Mk 1T armour-piercing shot (made of steel with an internal tracer). The standard HE shell remained the HE streamlined, Mk 1D, was used in conjunction with a variety of fuses and was usually filled with Amatol but could alternatively have TNT or RDX. The shells were normally painted to denote the type, buff being high-explosive, green for smoke and black for amour-piercing. Bands were used to denote special notification:
Head: Red - shell was filled Red crosses in a ring - a filled shell that was suitable for hot climates Black ring above Red ring - shell was fitted with exploders that were suitable for a powder-filled fuse White ring - Armour-Piercing round
Body: Green band - Shell filled with Amatol or TNT Two Black bands - An HE Practice round One Black band - A drill shell (empty) Yellow band - A practice projectile
The anti-tank round for the 25-pdr consisted of an armour-piercing solid shot with a copper driving band. Smoke and propaganda shells were base fused so they distributed their contents from the base. By the battle of El Alamein, the No. 210 fuse was available for air-bursts and was found to be effective against exposed artillery crews.

Organisation

By 1939, the Field Regiments of the Royal Artillery were the ones slated to receive the 25-pdr Mk I and were formed into two 12-gun batteries, each of three 4-gun troops. With three field regiments to a division, a division could theoretically have seventy-two 25-pdr guns. The smallest unit was of course the gun crew, known as the detachment, a term that goes back to the early history of the Royal Artillery when the gunners were formed into companies and then detached to their various guns. In each detachment there were six men, each numbered one to six:

No. 1 - the commander, made large traverses of the gun and normally positioned to the rear.
No. 2 - held the rammer, as well as operating the breech lever and stood to the right of the gun.
No. 3 - the 'Layer', sat on the wooden seat on the left-hand side of the gun, adjusted the sights, signalled adjustments to the No. 1 in big traverses and fired the gun.
No. 4 - the 'Loader'.
No. 5 - passed ammunition to the No. 4 and checked the fuses.
No. 6 - the second-in-command (2IC) who set the fuses and the charges, as well as being responsible for the movement and braking of the trailer.

Tractors

The Mk I was initially towed by the Dragon series of artillery tractor, manufactured by Vickers Armstrong, with some going to France with the BEF, probably the Light Dragon Mk IID. Being reliable and robust, they had been developed in the early 1930s to serve with the Royal Artillery. Another tractor utilised early on to tow the 25-pdr was one developed by Morris in 1936, the CDSW 6x4 field artillery tractor. However, the Army wanted to standardise on a 4x4 and such a specification was issued in 1937 with three companies involved: Commer, Guy and Morris Commercial. The most widely used was the Morris C8 / FWD Field Artillery Tractor (FAT) and became known as the Quad. It went through three marks and a bodywork redesign during World War Two. The Mk I had an enclosed body (having a metal roof) and the Mk II had an open top (having a roll-up canvas roof spread across metal bars welded across the roof). Guy Quad Ants were sold to the British Army but most were lost in the battle for France. Canada Ford and Canada General Motors also produced a FAT of the Canadian Military Pattern that was widely used with the Mk II and Mk III 25-pdrs.

Ammunition Trailers

25-pounder field guns, Knightsbridge Area, battle of Gazala
25-pounder field guns, Knightsbridge Area, battle of Gazala
The small trailer that usually appears near the 25-pdr when deployed or linking the gun and tractor when on the move was the ammunition trailer or 'limber' as it was known. In fact, it contained not only ammunition but a selection of stores as well. There were two types - the No. 24 and No. 27. The No. 24 was developed before World War Two and was replaced by the No. 27 after war had broken out. The No. 24 consisted of an ammunition box, axle tube, perch and towing arrangement and brake-operating gear. It carried thirty-two rounds of ammunition (the same as the No. 27) but was lower in height, had no provision for carrying the traversing platform and only had a wooded box on its top to carry tools with. Ammunition was carried in No. 5 ammunition trays made of galvanised steel, with wood felt and steel linings. The No. 27 trailer had more space for tools and spares and if needed could carry a firing platform on top. A steel storage tray was fitted on the front of the ammunition box that gave extra flexibility and could hold axes, crowbars, spades, the projectile ejector, handspikes, a jack, drag ropes and an illuminating point with a post spring and cover. The ammunition trays were similar but of a later design that completely encompassed the projectile and propellant.

Bibliography and Further Reading

Falvey, D., A Well-Known Excellence: British Artillery and Artillerymen in World War Two , London, 2002.
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Henry, C., The 25-pounder Field Gun 1939 - 72 , Osprey Publishing, Oxford, 2002, New Vanguard Series No. 48.
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Hogg, I V., Allied Artillery of World War Two , Allied Artillery of World War Two, Crowood Press, Marlborough, 1998.
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Hogg, I V., British and American Artillery of World War Two , Greenhill Books, London, 2002.
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Hogg, I V., The Guns 1939 - 45 , MacDonald / Jane's Publishing, London, 1970, Purnell's History of the Second World War, Weapons Book No. 11.
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The author would like to dedicate both articles on the 25-pdr Field Gun to his Granddad, Ted, who served in the Royal Artillery in World War Two as a Forward Artillery Observer attached to the 10th Indian Division.
How to cite this article: Antill, P. (14 January 2004), The 25-pdr Field Gun 1939 - 1972: Part One, http://www.historyofwar.org/articles/weapons_25pdr1.html

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