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The Light Gun v. the Twenty-five Pounder

A comparison between the 105-mm 'light gun' and the 25-pounder, by WL Ruffell

'Light' is the name given to the British 105-mm field guns supplied to 16 Field Regiment RNZA in 1987 by Australia where they were made under licence. Having examined one of the guns I could not help comparing it with the 25-pr.

Light gun on tow and rear.

Although the 25-pr became a legend during World War 2, you will recall that its calibre and performance were dictated by a parsimonious Government rather than the operational needs of the Regiment. Also, other aspects of its design left much to be desired. Consequently at the end of the war Gunners were still seeking the kind of gun they had sought before the war started.

In no way are the above remarks meant to denigrate the designers of the equipment who were working under some pressure with a war clearly about to take place. After all, they produced in under four years a gun which normally would have taken twice that time to bring into use.

In 1952 I remember the Gunnery Staff at the School of Artillery Larkhill (now the Royal School of Artillery), where I was then a student, spelling out the 25-pr's shortcomings, once again describing the type of field gun they considered essential for the Gunners' role in modern warfare. Very briefly it was to have a calibre of around 3.7 inches (94 mm), and a range of at least 15,000 yards (13,716 metres). It had to be capable of firing at high angle, i.e. above 45° (the 25-pr's limit) with standard sighting gear, without having to dig the trail down, or hinging the trail as in the Mark 3 version. The sighting system had to be much simpler than that of the 25-pr which was considered far too complicated, difficult to adjust, and possessed of inherent inaccuracies.

The carriage had to be just as robust but more versatile, e.g. easily broken down for carriage by air, and if possible no heavier than its predecessor. Clearly, to produce such an equipment a major research and development programme was necessary.

But during the years immediately following the war finance for new defence projects was at a premium, so as an interim measure the British Army in 1960 (after a two-year trial period) adopted the Italian-made Model 56 105-mm pack howitzer firing the American 105-mm M1 shell.

Altogether in the late 1950s and early 1960s twenty-one other countries also adopted the howitzer, following high-pressure salesmanship by the Italians who sent a well-trained detachment half-way round the world to demonstrate their product to interested parties. The New Zealand Army adopted the howitzer in 1964.

However, the equipment satisfied only some of the criteria outlined at Larkhill: its extreme range of 10,000 metres fell far short of the minimum Gunner experts felt essential. A single layer could not lay the howitzer; two men were needed, using a system reminiscent of World War 1 equipments long-since declared obsolete. The carriage lacked strength; stub axles failed, so that on long hauls at high speed the howitzer could not be towed but had to be carried on the back of a truck. In Vietnam elevating gear-boxes cracked. To be fair to the Italians the equipment was never meant to stand such treatment; it was designed to be 'packed' in mountainous areas typical of Italy.

The ammunition, the American 105-mm M1 shell, was not as lethal as the 25-pr; the latter is a very efficient man-killer indeed. On detonation it breaks up into about 500 splinters each of which will kill or maim a man within splinter range, plus a number of smaller fragments each of which will inflict a nasty wound. Some Gunners thought we would have been better off staying with the 25-pr.

They saw the 105-mm howitzer as a possible replacement for the ML 4.2-in mortar, but not as a suitable successor to the 25-pr. Commando Light Batteries RA remain equipped with the howitzer.

In the meantime, to replace the 'Sexton' (25-pr on Sherman chassis), in 1965 the Royal Horse Artillery were issued with a new British-made 105-mm self-propelled (SP) gun which they called the 'Abbot' (shown) Abbot in deference to its ecclesiastically-nicknamed forbears the 'Bishop' and 'Priest' 25-pr on Valentine and U.S. 105-mm gun on Sherman chassis respectively. The gun was mounted upon a derivative of the armoured personnel carrier (APC) FV430 series chassis. It had a range of 17,500 metres and fired a shell significantly more lethal than the U.S. 105-mm M1 at a maximum rate of 12 rounds a minute. Having given the RHA a gun of such power the British Army now set about providing the field artillery with one of similar performance. The light gun was the answer.

Development of the new gun began in 1966, the first user trials were held in 1971, the gun was formally accepted three years later, and by 1975 had been issued to most RA units.

Decision on the calibre is said to have been reached 'by agreement' with the other members of NATO, most if not all of whom were already equipped with field and/or SP guns of 105-mm. Although experience with the 25-pr (87.6-mm) and the German 88-mm guns during World War 2 convinced many experts these were best for field artillery projectiles. Adoption of a new gun of, say, 87.6-mm calibre would have left Britain out on a limb in the event of a major conflict, especially when it came to ammunition supply. So she was obliged to fall into line and adopt the 105.

To digress a moment, before the decision was taken the United States and other NATO countries favoured six-gun batteries. One of the arguments advanced by the Americans in favour of a 105-mm (33-lb or 15 kg) shell was that six of these projectiles deposited on a target the same weight of metal as eight 25-prs. From a simple arithmetic point of view this argument is near enough correct, but it ignores the primary role of field artillery which is the close support of Infantry. The man with rifle and bayonet well knows that eight shells on a given area will keep more enemy heads down than six - even if they are a few pounds lighter.

The new equipment comes with two barrels; that normally fitted for service fires the same ammunition as the Abbot with a like performance, i.e. an extreme range of 17,500 metres. For training purposes, and to use up the large quantities of American M1 shell supplied for the pack howitzer, a second barrel is provided which gives a range of 11,000 metres. A barrel can be changed in about two hours.

Click image to enlargeBarrels are auto-frettaged throughout, i.e. pre-stressed. Each is fitted with a muzzle brake easily removable for cleaning, but with no securing screw likely to cause a dangerous constriction in the barrel, as sometimes occurred with the 25-pr.

Each barrel is fitted with a closed-jaw breech ring, not the open-jawed type as found on the 25-pr. The latter tended to weakness in the internal angles (which had to be modified by radiusing during World War 2 to prevent cracking), and I know of one which failed after the war while firing super plus increment, killing two members of the detachment. With the closed-jaw ring the firing mechanism is necessarily totally enclosed within the breech block, but is no more difficult to dismantle or assemble. Firing is electric with Abbot ammunition but by percussion with American M1. The LBM in each case is mounted on top of the breech ring to make opening and closing of the breech easy at all angles of elevation.

To permit operation at high angle (the light gun elevates to 70° or 1244 mils), rear trunnions are fitted, a situation which calls for a gun-balancing gear, a simple spring tension type requiring virtually no maintenance. A buffer cut-off gear similar in principle to that of the 25-pr shortens recoil as the gun is elevated, thus preventing the breech from striking the ground.

Sighting arrangements are very simple; the layer has his dial sight, cross-levelling gear, QE scale with accompanying bubble, plus a telescope for anti-tank action. The sight carrier is attached directly to the left trunnion. There is no sight clinometer, separate range scales, MV correction plates, drift scale plate, TE scale - nor complicated gearing and linkages to get out of adjustment. All range data come from the Command Post in the form of quadrant elevation (QE). Gone is that equipment designer's abortion the 'apparatus illuminating sights'. All scales and graticules are permanently illuminated by a nuclear light source; there are no batteries to worry about.

Now that we have changed over to the metric system all scales are in mils. The true mil is one-thousandth part of a radian, i.e. there are 6283 in a circle, but for military purposes the circle is divided into 6400, a figure more conveniently divisible. Thus one military mil equals 3.375 minutes of arc.

The light gun carriage has a box-type trail, the side members being of tubular section and bow-shaped, enabling both layer and breech number to remain within the trail yet be clear of recoil. Top traverse is 5.5° (100 mils) right and left. In action the 900 x 16 wheels run on a circular platform resembling that of the 25-pr, except it is more robust. It is connected to the underside of the carriage by three short wire ropes instead of rigid metal stays. For travelling it is carried on top of the trail.

In the field the gun may be towed in the normal position but for long-distance travel it is traversed through 180° and the muzzle clamped down over the trail eye. This 'folded' position makes a more stable load behind the towing vehicle as well as a compact load for air movement. Traversing the piece requires the removal and replacement of the right gun wheel, which is fitted with a quick-release for the purpose, but can be done by a trained detachment in about two minutes. In New Zealand the normal towing vehicle is the Mercedes 1.5-tonne UNIMOG, in the UK the 1-ton Landrover with V8 engine.

The complete equipment can be lifted by Chinook, Puma, and Sea King helicopters, none of which are available in New Zealand. To be lifted by the Wessex the carriage and elevating mass are separated and re-assembled in the field. Re-assembly takes only ten minutes using a single simple tool.

Probably the most impressive feature of the light gun when compared with the 25-pr is its weight. Despite its much greater power (muzzle energy/weight = 640 against 280 for the 25-pr), the light gun is only about 27 kg (60 lbs) heavier, an achievement due principally to the use of a very high quality steel developed by Firth Vickers, plus the employment of modern techniques of construction and testing. These include the explosive forming of complex shapes of steel sheet, a method cheaper than pressing.

However good a gun may be it is the shell which is the weapon. Projectiles include HE, carrier, canister (anti-personnel up to 500 metres), HE squash-head (HESH) for use against tanks up to 1200 metres. HE may also be used against tanks up to 2500 metres. Carrier shell are ballistically matched to the standard HE, so Command Post staff do not need to work out 'abnormal projectile' corrections as they did with 25-pr. The Abbot shell (filled RDX/TNT) is significantly more lethal than the American M1.

Fuze types include percussion, time and percussion (with mechanical time mechanisms) and proximity.

Maximum muzzle velocity with Abbot ammunition is 709 m/sec (2325 feet per second) against 520 m/sec (1706 f/s) for the 25-pr.

Ammunition is 'separate' so the loading drill will be quite familiar to 25-pr men. There are seven charges, 1 to 6 plus super. Range with Charge 6 is 15,000 metres, and with Super 17,500 using Abbot ammunition. With US M1 ammunition extreme range is only 11,000 metres - with its own set of charges. Normal rate of fire is 3 rpm, intense 6 rpm.

With the 'base bleed' round range is 20,000 metres. In this a device within the base of the shell generates a gas pressure which tends to overcome the vacuum formed at the rear of a shell in flight, thus neutralising to some extent 'base drag' and so increasing range.

To conclude, here is some food for thought: when the Royal Artillery received the first of their light guns in 1975 the price in the UK was £43,000 per complete equipment including spares back-up. Twelve years later in 1987 when the New Zealand Government bought the same guns from Australia (where they are called 'Hamel' guns), the price had climbed to $NZ1,000,000 - yes, one million dollars - for the same package. No doubt we were paying for Project Hamel, the four-year tooling-up exercise by Australian ordnance factories - or did the Aussies see us coming?

WL Ruffell, 1987

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