Cold War, Experimental, United Kingdom

Britain’s Monstrous Green Mace AA Gun

Today’s subject is going to cover the Green Mace anti-aircraft gun, a rather odd project, one that had a lot of potential, but found itself in ever-increasing difficulties. It’s a gun system that could rip apart anything it chose and could fire more fin-stabilised discarding-sabot rounds than an entire tank regiment in a matter of seconds – this is the story of the Green Mace anti-aircraft system.

To understand Green Mace we have to go back to the Second World War, which had ended the year previously.



The British had a surprisingly effective number of anti-aircraft (AA) weapons, which can crudely be broken down into three types.

First is LAA, or light anti-aircraft, such as the Bofors and Vickers 40 mm and the odd 6-pounders.

Then there was the heavy anti-air (HAA), ranging from 3 inch (76.2 mm) to 5.25 inch (133 mm) guns. There was also a vast amount of rocket batteries of various designs which had mixed results and ended up killing more British civilians than the German bombers they were engaging.

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These weapons were generally a mixed bag in terms of effectiveness. They accounted for a higher ratio of kills than German AA guns due to devices like the Vickers predictors, reliable proximity fuses and the excellent use of radar, which all helped to force German bombers to fly higher and wider to avoid flack. Yet what goes up must come down, and studies show that many civilians lost their lives to falling munitions.

Post-war however, aircraft from the Soviet Union were now the principle threat, and it was quickly decided that newer platforms would be needed.

Brakemine missile on its launcher.
The Brakemine missile, one of Britain’s first guided missiles.

The war had shown that bombers were increasing in size, payload, and speed as well as the altitude they could fly, and this required weapons that could not only engage high-altitude bombers but also fast-moving jets and ground attack aircraft.

Thus the UK set about an extensive overhaul of both systems.

They had been working on guided missile systems since the mid-1940s, but these were expensive and still in their infancy. The War Office had a habit of siding with tried and tested where possible.

New Developments

The LAA program saw several designs and developments, such as the Red King and Red Queen 42 mm weapons. Red King was from Bofors. Red Queen was by the Armament Design Establishment (ADE), beginning in early 1949 and continuing into the early ’50s. There were also radar tracking systems, such as Orange Pipping and Red Indian.

Ultimately these weapons would not progress, as while they had a very high rate of fire, the cost in size, weight, and price, as well as an engagement range of just 10,000 ft, was deemed unacceptable. Instead, the Bofors L70 would become the standard LAA platform.

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Meanwhile, on the HAA front, a new program was started under the name of Rate Fixer, which aimed to enhance the current weapons in ranges, rate of fire, and lethality.

3.7 inch AA gun.
A 3.7 inch anti-aircraft gun, Britain’s main heavy AA weapon of the Second World War. Image by Hugh Llewelyn CC BY-SA 2.0.

This led initially to five systems being put in place: a modified 3.7 inch AA gun with a high-speed loading system, K, C and CR which featured hopers and drums, or a belt feed, and the Ratefixer CN by Frazer Nash that worked with a belt feed and utilized hydraulic feed systems.

However despite some working prototypes, and plans to mount these on vehicles such as the FV201 and FV300, issues around stability and size became problematic.

Work was ramped up as international tension with the Russians and the outbreak of the Korean led to a fear of high-speed nuclear-armed bombers at high altitudes – something the 3.7-inch guns were simply unable to engage due to the long flight time and availability of the Soviet planes to evade fire in the travel time of the round.

3M Bison in flight.
Newer Soviet bombers were capable of flying almost twice as fast as 1940s-era fighters.

This led to a further five new designs for larger guns. The first was a 5.25 inch gun with Probert rifling; a smoothbore tube with only partial rifling, that gives the round spin but then smooths out the driving bands to reduce drag on the round.

Two designs were for a squeezbore concept – one 4-inch to 3-inch by the ADRE, and one 4.26-inch to 3.2 inch by Vickers. A third by the Chief Engineer Armament Design (CEAD) utilized a 3-inch gun with a 2.32 inch core, firing fin-stabilized rounds with a variable time (VT) fuse.

Finally there was a 5 inch to 2.78 inch fin-stabilised discarding-sabot (FSDS) with direct action (DA) fuse.

These latter FSDS rounds had already been in testing since 1948, after several captured examples of German FSDS rounds had been brought back to the UK.

German sabot rounds.
1940s-era sabot round.

Interestingly enough the UK had worked on its own armor-piercing fin-stabilised discarding-sabot (APFSDS) rounds during the Second World War under Sir Dennison Burney. These were named Duplex rounds, but were forgotten about post-war.

Of the proposed guns, the Vickers 4.26-inch squeezbore and the 5-inch 2.32 FSDS gun were selected to proceed further.

By 1951 progress with the 4.26 inch Vickers gun was not showing much promise, so a new 4-inch 102 mm gun with a skirted band projectile began development. This gun was novel in being water-cooled and capable of extremely high rates of fire and velocities.

Green Mace Development

Meanwhile, the 5-inch gun would be provisionally accepted in 1952 under two names, Green Mace and Fixed Mace. Green Mace was the complete system, while Fixed Mace was a similar system but with a fixed mounting in just one azimuth for trials purposes.

Fixed Mace experimental system with 5 inch gun.
The Fixed Mace, fitted with the 5 inch gun.

By 1953 the 102 mm gun is also accepted to progress further and is given the designation X1E1 and mounted to the Fixed Mace firing platform under the new name Green Maid.

It undergoes its first live firing trials in 1954.

A year later in 1955 the 5 inch FSDS barrel is fitted to the Fixed Mace, and also named Green Maid.

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The 102 mm and 5 inch systems sharing the same name causes some confusion. The surviving gun is often said to be a QF 127 mm (5 inch) weapon, but it is actually the 102 mm.

From 1954 to 1955 another AA project, the X4 Longhand, is also under development. This involved a 3.7-inch Mk 6 gun with a 12-round automatic feed that stemmed from the Ratefixer CN program. In addition to this was the X4 Shorthand which had a manually loaded system. Work on these two guns is allowed to continue as an insurance backup in 1956 should the 102 mm and 5 inch experimental guns run into any problems.

102 mm Fixed Mace Green Mace.
Fixed Mace with 102 mm gun.

However, at this time the Royal Artillery decides to back out of the AA role, handing over most of the project to the Royal Air Force.

In 1956 the 102 mm gun is mounted onto the Green Mace mobile platform. However testing is less than stellar, with multiple failures occuring.

The weapon consistently jammed and bits fell off it.

Over 900 rounds were fired with almost 80 failures, for approximately 12 rounds per fault. This was deemed beyond the scope of a simple fix.

She was one chunky system coming in at 27.5 tons in travel position, and powered by two Rolls Royce B40 engines. But even with this she was barely able to move under her own power, so it was loaded on a 30-ton flatbed to move it around. It also required a large generator unit on site to provide power, and the wheels had to be removed to load it into its firing position.

Green Mace

So what was the system capable of if it worked? For this, there are two data sheets, listing the two different guns.

The 5-inch gun is recorded as holding 13 rounds per drum, one either side, with an engagement range of 25,000 ft. 30,000 ft was also possible, with a 9.7 second time of flight to reach this altitude.

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The rate of fire was recorded as a continuous burst of fire for 20 seconds, with a 20-second interval, for a total theoretical rate of fire of 80 rounds per minute – although in practice it would use its ammunition up before this.

Green Mace anti-aircraft system.
The Green Mace system, fitted with the 5 inch gun.

It appears from a later document that 2 rounds were made, a DA and VT.

To load the gun, the situation was a bit more complex; a second pair of loaders were to man forklift trucks, positioned to the sides. They would then drive up and align ammo stillages with the drums or hoppers, then get out of the cab and manually feed the rounds into the magazines while the gun was cocked up into a reload position while staggering their fire in a battery.

This meant that theoretically, at least one system was always in a reload cycle while others were firing. The gunner himself on both samples sat in a small cockpit to one side and would have been fed tracking data from a separate radar system.

Green Mace AA Gun.
The two large ammunition drums behind the gun on the Green Mace.

The 102 mm gun is recorded as having an elevation of 85 degrees, a range of 39,000 ft and a muzzle velocity of 3,100 fps. Bizarrely, it is also recorded as having a heavier round at 32 lbs for a total weight of 72 lbs per round and a rate of fire of 40 rounds per minute.

In 1957 the 102 mm X1E1 gun is abandoned and the simpler X4 longhand gun is accepted into service after passing its evaluation period, while work on projects like Red Heathen guided missile was also showing promise as a high altitude surface-to-air missile.

Green Mace would struggle on for just one more year with the 5-inch version undergoing firing trials as part of the Mace system before also being cancelled.

Green Mace with 102 mm.
Green Mace fitted with the 102 mm gun. This is example still survives today.

Incidentally, if one looks closely there was also what appears to be a small display model of another platform – this one has 6 roadwheels and a small cab to the front, so may have been the finalized vision for Green Mace.

Although not all was lost, much of the information gathered was later used in the design of the 3-inch N1 guns used on the Tiger-class cruisers, where they also didn’t work very well – but that was somebody else’s problem.

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Today the only surviving prototype is in stowage at the Royal Artillery Reserve collection pending a new home.