Birch Gun MK.I

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                              Birch gun MK.1 


Vehicle design and service history:

The Birch Gun’s inception began in 1923 when Sir James Frederick Noel Birch was appointed to the office of Master General of the Ordnance. Sir Birch was a former cavalryman but was also heavily involved with issues with artillery that had been experienced during the First World War, resulting in him being well-versed in the issues that had come with mechanization. This would make him a competent person to head a self propelled artillery project, which is exactly the specification Vickers had received in 1921 for a tracked self-propelled gun.

Work began on this project in 1923, just as Birch assumed his position, and in order to save costs , they decided to utilize the hull of the Medium Mark II which was in development at the time. Ironically the Birch gun pilot would be completed before the Medium Mark II’s, as it was completed in early 1924, due to the more simplistic design of the vehicle. This resulted in the chassis having some differences from the production Medium MK II, for example, the Birch Gun lacked side skirts covering its suspension. The number of road wheels and return rollers was also increased, along with differences in the drive sprocket and idler wheels in order to use the No.9 Link Track.

Most of these noticeable differences in the hull and suspension between the Medium Tank Mk.II. were done in order to accommodate the requirement of a 360 degree horizontally traversing gun. This would make the vehicle top heavy, and as such the height of the hull was reduced by lowering the engine in order to bring the centre of gravity closer to the ground. Additional weight-saving measures involved the driver losing his armoured cabin, instead replacing it with a movable shield. The lack of a turret also allowed the fighting compartment height to be reduced, with the gun positioned in the centre of the vehicle, again lowering the centre of gravity. The crew would then man this gun from an enlarged turret ring, which in turn resulted in an enlargement of the hull at the top in order to accommodate it. This choice allowed the Birch Gun’s crew of six to be comfortably positioned even with the limited room.

The gun itself was a new version of the 18-pdr (83.8 mm) field gun designed just for the Birch gun. If differed in its mounting which allowed for exceptional elevation so that it could engage both airborne and ground targets. This change resulted in a gun completely re-engineered with the exception of the barrel, and it could fire HE, Shrapnel, AP, AP-T and according to some sources APHE shells allowing it to engage a wide number of possible threats.

In this form the experimental Birch Gun would then enter trials in 1924 with the 28th Field Battery of the IX Field Brigade of the Royal Artillery. With these trials, a list of improvements was put forward, which would ultimately culminate in the Birch gun MK.1E, but that is a story for another suggestion. The trials produced several things of note, the most pressing negative being the fact the SPG was effectively devoid of armour, though they did not ask for a complete casemate, instead content with a gunshield, which would be added to the pilot and other MK I’s when in service. Another request was skirt armour to protect the vulnerable suspension elements. The army seemed happy with the result though, and Vickers received an order for four 18-pdr SPGs in September of 1925. This order would be fulfilled in about a year, with each vehicle coming to the cost of 12,250 Pounds Sterling, or about 1000 Pounds Sterling per ton. The Birch gun would weigh 12-tons and have at most 5mm of armour, whilst retaining the engine and transmission of the Medium Tank Mk.II.

As requested these vehicles would receive the large gun shield, which completely covered the crew from the front, and a travel lock was added. The gun elevation was also increased to 80 degrees, along with the ammunition rack increasing to 80 rounds safely stowed in the hull. Additional testing with these vehicles showed their drastic improvement over towed guns, as it took but seconds for a Birch gun to transition from travel mode to firing mode, a task that could take upwards of a minute for a towed gun. This would continue in additional trials and fire exercises, as the Birch gun quickly showed it had no equals, though this did not stop it from receiving negative feedback. The largest ones are the still high center of gravity on the vehicle making it top heavy, along with the kickback of the 18-pounder being significantly more than the 3-pounder causing damage to the suspension over time. The lack of armour was also noted, which was unsuitable for engaging enemies who were liable to shoot back.

Tweaks to the design would take place, but the Birch gun would start to lose traction when in 1927 Sir Birch left his post and transferred to Vickers. His replacement was more conservative in his battle doctrine and believed that artillery should be pulled by tractors. This in tandem with the negative feedback further weakened the Birch gun’s position, and the Experimental Mechanized Force would be disbanded in early 1929 effectively closing the Birch Gun program.

This would basically mark the end of SPG development in the UK until the second world war, which though at the time heralded as a victory for conservatives would be a great detriment to the British war effort. To the surprise of the old guard the self-propelled artillery would prove vital in the mobile warfare of the Second World War, both experienced in the fall of France and the later war in North Africa, as the modern battlefield was significantly more mobile than they had envisioned. This would result in the artillery programs starting effectively from scratch, and in some cases even resulted in the same mistakes. This caused British SPG’s to be often unconventional in layout, and simply inferior to foreign vehicles of the same class, with the best and most prolific designs based on American chassis and spgs…

Vehicle specification:

Mass 26,700 lb (12,100 kg)

Length 19 ft 3⁄8 in (5.80 m)

Width 7 ft 10+1⁄2 in (2.40 m)

Height 7 ft 6+1⁄2 in (2.30 m)

Crew 6

Armour 6 mm (0.24 in) Steel

Main armament 1 × QF 18-pounder 3.30 in (83.8 mm) gun (HE, AP, AP-T, Shrapnel and possibly APHE)

Engine 1 × Armstrong Siddeley 8-cylinder petrol engine 90 hp (67 kW)

Suspension bogie

Operational range 119 mi (192 km)

Maximum speed 28 mph (45 km/h)

Additional historical photos:



A fantastic option for a low-tier SPG. +1

why germany?

Cool vehicle +1

you are correct why is it germany…
I completely missed that also*

+1 for a low tier British SPG. Here is some info on the ammo:




So for the APHE (!!!) specifically, it would be a 8.39kg shell with 0.25kg of explosive filler (Shellite) with a velocity of 492 m/s.

I’ve tried a few of the DeMarre/ Krupp calculators and it seems this would give it a point blank pen somewhere in the region of 60-70mm? If this is the case, this would make a really fun and unique 1.0-2.0 SPG.

Terrific find! Good data on it as well, fantastic! Where did you find this?

On the penetration, I popped it into the WT calculator and got just shy of 60.5mm pen. Decent for low tier and a filler charge to boot!

Handbook for the Ordnance QF 18 pdr - 1940

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