Why did Spitfires change their guns?

Liam Shaw: "Behind me is one of Imperial War Museums' most prized possessions. A genuine airworthy, combat-veteran Spitfire Mk 1. It was examples like this alongside the Hurricane, that fought the Luftwaffe during the summer of 1940 to keep Britain in the war.

Early Spitfires like the example behind us were initially equipped with eight machine guns. They fired the .303 rifle caliber machine gun round. But much like the Spitfire through its development, not only did it gain things like bigger engines, but also larger armament. In the case of the 20mm cannon and the large Caliber .5 inch machine gun.

The story of the Spitfire's armament development is complicated and can even be considered controversial to this day. To help us tell that story we've brought in Jonathan Ferguson of Royal Armouries who's going to take an in-depth look at the three main weapons that the Spitfire would be equipped with throughout the war. But first let's step back and look at the development of the very first Spitfires."

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Liam Shaw: "During the First World War air fighting was in its infancy. Aircraft were developed to carry different ranges of weapons, but even at their peak most fighter aircraft only carried a maximum of two forward firing machine guns. During the interwar period planners began to look forward to a potential future conflict and what might be required. Aircraft were developed and it was deemed that a minimum of four machine guns would be required in any future war. This is evident in the last RAF biplane to enter service, the Gloster Gladiator, equipped with four forward-firing 303 machine guns.

But even as the Gloster Gladiator is entering RAF service, the next generation of fighter aircraft is just over the horizon. These aircraft will be monoplanes and they will be fast, capable of flying over 300 mph. The Air Ministry realised that these aircraft may only be capable of following an enemy in combat for around 2 to 3 seconds, only a very short period of time to actually down that enemy aircraft. So enter into our story Fred Hill and his daughter Hazel who calculate that, to put down this required amount of fire, we don't need four machine guns but we need eight.

Although eight machine guns seemed excessive at the time the most cutting edge fighter just about to enter RAF service, the Spitfire, was able to accommodate these machine guns. With four in each wing spread throughout. So now over to you, Jonathan, to tell us more about the 303 Browning machine gun."

Jonathan Ferguson: "Hi guys Jonathan here at the Royal Armories with our wonderful collection of firearms, of course. This is technically a firearm even though it's mounted in an aircraft, specifically the Spitfire. This is the first and probably most iconic gun used in the Spitfire and the Hurricane of course and other aircraft. This is the Browning, this is a Mk 2 Star, 303 belt-fed machine gun, air cooled, you basically have to go air cooled with aircraft guns.

This thing is putting out 1,150 rounds per minute. That's a pretty high rate of fire, good chance of tagging the enemy with some of those shots. The drawback was the round. This is a rifle caliber round. More than capable of killing a person, of course, multiple people potentially. The problem was the size of it. It can only be this big which means you cannot fit any explosive content in there to really do damage to the fabric of the aircraft. What you're doing instead is putting a sort of swarm of angry bees into the enemy and hoping that you hit something vital.

Pneumatic firing system you can see our cables attached to this one, two required for the Browning. A very capable gun, you've got eight on the Spitfire. 300 rounds per gun initially, that's a lot of rounds at 1150 rounds per minute to hopefully hit something. But you might find you're hitting, you're hitting, and you're not doing the damage that you really need to do to knock that aircraft out of the sky."

Liam Shaw: "By the time the Spitfire comes into service it's equipped with eight 303 machine guns, four in each wing. But with the increasing speeds and altitudes these aircraft can fly at some problems come about. One of which is the guns themselves freezing as the aircraft fly at these heights. To combat it, hot air is pumped from the engine, through the gun bays to keep them warm and also patches are fixed to the front of the gun apertures to stop moisture and dirt getting inside.

As such, the Spitfire's armament is ready to face its biggest test. Come the summer of 1940 and what we now know as the Battle of Britain. Following the fall of France the German Luftwaffe attempted to gain air supremacy over Great Britain as a prelude to invasion. The three main single engine fighters of this time were, on the British side, the Spitfire and the Hurricane equipped with eight 303 machine guns. On the German side, the Luftwaffe equipped with the single engine Messerschmitt BF-109 and in its latest versions it's equipped with two machine guns and crucially two cannon. These cannon are able to fire explosive shells.

Because the British fighters were equipped with eight machine guns they had a higher rate of fire, meaning they could fire more bullets at the enemy in a certain span of time. However the differing range of weapons the Germans had meant they had a higher weight of fire, meaning that they could put down an increased volume of ordinance over their British counterparts. So it's clear that this is all about balance. What is the right combination to use? And it was something that was debated heavily at the time on both sides of the opposing forces and is still debated often with much controversy to this day.

But something was changing the calculations on the British side. That was the increasing amount and prevalence of armour in the German bombers. The hitting power of the 303 was not fantastic and its penetration ability of this armour was weak. meaning that a higher volume of bullets needed to hit the aircraft have any chance of bringing it down. As such the British require a weapon with more hitting power to bring down the enemy aircraft, so they begin experimenting with a new weapon, the 20mm Hispano Cannon. And for more information on that, over to you Jonathan."

Jonathan Fergusonl "Right, now the RAF takes an enormous leap forward from the tiny little, relatively speaking, 303 rifle round with the Hispano 20mm. As you can see these are markedly larger, much bigger and heavier, even if you were firing standard lead cord ammunition, ball ammunition. The beauty, if you can call it that, of these is that they can contain really large armour piercing penetrator which is what one of these guys is. So we've got semi armour piercing incendiary, that's the the red with the white tip. And then next to it we have the high explosive incendiary that's tearing apart, tearing big holes in the fabric of the aircraft, ripping control lines, tearing through hydraulics, fuel tanks, you name it. That powerful one/two punch is how these belts were loaded. with these two types of round.

The gun itself is quite technically interesting, we won't go into that here. It's gigantic, which is why you can't see the front end. Now the front end on this is set up for a different aircraft, very late war or post-war. The gun itself is a Mk 2 exactly as used in the Spitfire.

These big rounds are belted together and initially fed from this 60 round gigantic magazine that the armourers had to hike out of the wing and replace. Later they're replaced with 120 rounds belted, feeding through a belt drive mechanism which is what this is. So that stays on the gun and the belts are just loaded straight into the wing bays in the ammo containers and they just feed straight through. So you go from 60 to 120 and then later on when we have the big Brownings alongside this you get to 135 rounds per gun. Rate of fire, about 800 rounds per minute. For the most part you're only going to be having two of these on your aircraft. So initially 60 rounds per gun, two cannon initially, just those, which doesn't go that well. Then we have augmented with machine gun ammunition as well and that's, that's the sweet spot for the RAF and for the Fleet Air Arm is to have a mixture of faster firing smaller caliber projectiles and very slow firing hard to tag the enemy with, but devastating if you do hit them and that's the mix that works."

Liam Shaw: "The first ever Spitfire Squadron, 19 Squadron, was formed at RAF Duxford before the Second World War. During the Battle of Britain it moved just down the road to RAF Fowlmere where it becomes the first Spitfire squadron equipped with cannons. But the Spitfire had an incredibly thin wing which was a problem when it came to fitting cannon armarment into it. The cannon is a larger weapon than the machine gun, it has drum-fed ammunition, and that required the cannon to be mounted sideways in the wing to get it to fit. When these new weapons worked in the Spitfire they increased the punch that the aircraft could deliver. Unfortunately, they rarely ever did work.

The G forces on the aircraft and the stresses in the wing often caused the ammunition feeds to fail and the guns regularly jammed, sometimes firing as little as one or two rounds. 19 Squadron persevered with this new armament, but were more than fed up. So much so that they requested their old Spitfires back. They were actually happier to have clapped out old Mk 1 Spitfires than their shiny new Mk 1s with the cannon."

George Unwin, 19 Squadron, RAF: "Unfortunately they they hadn't got the secret of belt feed mechanism and we had a little magazine which only carried 60 grounds in each gun which lasted 6 seconds. And unfortunately if you applied any G, due to the magazine feed, they stopped. And we kept protesting because these things should never have been issued in that state. We were just furious that we should be issued with stuff that had obviously never been tested. For a long time,  matter of weeks with these damned aeroplanes."

Liam Shaw: "As such experimentation with cannon armament ceased during the Battle of Britain. Cannon armament would only really gain maturity in the wing of a Spitfire by the time we get to the Mk 5 variant.

We've moved now to another hanger here at Duxford, hanger 2. A hanger that is regularly full of airworthy Second World War aircraft. There is a bit of noise in the background today because it is a very windy day here at Duxford. But we are now with our next subject Spitfire Mk 5c, JG891 of Comanche Fighters. From The Mk 5 onwards the Spitfire began to be fitted with a variety of different wings. In the case of the Mk 5, three different wings the A, the B, and the C.

First we have the A wing which we've already seen on N3200 in hanger 4 which is only capable of carrying eight 303 machine guns. Then there was the B wing, like here on MH 415. The B wing was only capable of carrying two 303 machine guns and a 20mm cannon in each wing. And you can tell the B wing by the fact that there isn't a separate fairing for an additional gun and it's got a smaller blister above the cannon barrel.

And finally we have the C wing, or the Universal wing. A wing that is capable of carrying a variety of weapons depending on its role, either a pair of 303s or a pair of 20mm cannons in each wing or a combination thereof. The C wing or Universal wing was incredibly important to Britain's war effort because it sped up manufacturer and was far more efficient in its production.

This version of the C wing has a pair of 20mm cannons, but that was quite a rarity. Most C wings in service were equipped with two 303 machine guns and one 20mm cannon, much like the b-wing. And part of the reason for keeping that layout was that pilots like to be able to sight and attack their target initially with machine guns where they had more ammunition, before they open fire with the hitting power of the cannon where they had less shells.

As the war progressed the Spitfire needed to be upgraded to meet constantly updating threats from the enemy. In the case of the Mk 5 it was being bested by the German Focke Wulf 190. New aircraft needed to be developed and that aircraft would be the Mk 9. From The Mk 9 Spitfire onwards the designation A and B were largely dropped in favour of the C D and E wing.

Although a large number of the Mk 9s were equipped with the C wing, British pilots still felt that as enemy aircraft were developed they still lack the punch that they needed with the 303 bullets and that led in the Mk 9 to the development of the E wing. A wing that was capable of carrying a 20mm cannon and a .5 inch heavy machine gun round. So, over to you Jonathan for one final time to tell us about the .5 inch machine gun."

Jonathan Ferguson: "What we have here might look like a giant 303 Browning and it basically is a giant 303 Browning. It's the same design scaled up to the 50 caliber, the 50 Browning machine gun round still in common use today and it is absolutely enormous. At this time it's a very potent aircraft gun used in virtually all of the US fighters and the Spitfire with its new E wing in 1944, late 44, gets the ability to mount two of these alongside the Hispano cannon giving it a fair bit of punch.

If we're comparing that to the other Browning round the 303 and it's almost hilariously much, much bigger. It's 800 rounds per minute, right between the two other guns. And it is 260 rounds per gun, but you only get two guns so you're still a bit low on the ammo count. But it's a great compromise for rate-of-fire and fire-power, because this big old round here it can be armour piercing, it can be high explosive, it can be armour piercing incendiary which is a common aircraft round. So, many would say this is the perfect compromise, the Americans notably. The Brits went, still compromising, with this and the Hispano for the later marks of Spitfire."

Liam Shaw: "So we've heard about a number of wings the Spitfire was equipped with the A, the B, the C, and the E, but now we're going to go back over and have a close look at a very special Spitfire that equipped with a D wing.

So we've come through to another hanger here at Duxford, hanger number 3 to take a look at another version of the Spitfire, this time a Spitfire with the D wing. The D wing actually means it's got no armament because this is a photographic reconnaissance Spitfire PR Mk 11, PL983.

Very early on in the Second World War aircraft were required to take pictures of the enemy, become spies in the sky, and the Spitfire was the fastest aircraft the RAF had available became very useful in this version. For the photographic reconnaissance Spitfires, in the place of the ammunition and the guns in the wings, fuel could be added which allowed these short range fighter interceptors to have the legs to reach out over the enemy territory and take photographs as an aerial camera platform. If these aircraft were intercepted by the enemy they relied on their speed to evade interception. These Spitfires were the fastest of all Spitfires, as they didn't have the weight and the drag that came with armament in the wings.

Aerial reconnaissance was incredibly important during the Second World War, developed by all sides, but particularly so by the British and Americans and really it's photographic reconnaissance that is the backbone to what becomes the Allied strategic bombing campaign. The aircraft behind me is a Spitfire PR Mk 11, PL983 belonging to the aircraft restoration company. This particular aircraft was assigned to number 2 Squadron in early 1945 and is known to have flown at least one high level photographic reconnaissance mission in around April 1945. And the names are part of a sponsorship campaign to raise money for NHS charities.

So the aircraft behind me is one of many Spitfires that you can see at Duxford on a normal day, covering the majority of the variants and marks that were produced. From The Mk 1 through to the final Spitfire variant the Mk 24. This particular variant was equipped with an entirely redesigned wing, exclusively holding four 20mm cannons. The armament that would take the RAF into the jet age."