Sorting Out the "E"

American Armament for the Spitfire Mk. IX/XVI

By Martin Waligorski

Browning .50 calibre M2 aircraft gun. Profile view and cross-section
[USAF]

Click to enlarge image

The mass-produced Spitfire Mk. XVIE incorporated two important items of American origin - the Packard-produced Merlin 266 engine and the .50" calibre Browning wing armament. By early 1944, both became available in large quantities from the United States through the increasingly reliable Atlantic shipping routes. Their incorporation in the production of the most numerous British fighter was the logical consequence of this supply situation.

The Browning .50" calibre machine gun had a long history. Even today it ranks as one of the most successful machine guns ever produced. It was used extensively as a vehicle weapon and for aircraft armament by the United States from the 1920s, through World War II to Korea and Vietnam. Surprisingly, it is still in use today as the primary heavy machine gun of the US military and NATO countries, with only a few modern improvements distinguishing it from its WW2-era predecessor.

The basic M2 was deployed in US service in a number of variants. The full designation of a variant dedicated for use as fixed or flexible aircraft gun was Browning Machine Gun, Aircraft, Cal. .50, AN/M2 (Fixed) or (Flexible).

The AN/M2 (fixed) had a cyclic rate of 750–850 rounds per minute, with the ability to be fired from a electrically-operated remote-mount solenoid trigger. Cooled by the aircraft's slipstream, the air-cooled AN/M2 could be fitted with a substantially lighter barrel than its Army counterpart, which also increased the rate of fire.

AN/M2 Fixed
[USAF]

It should be noted that compared with other aircraft weapons of the day, the performance of the Browning was rather undistinguished, especially in comparison with aircraft cannon widely used by other combating nations. The American gun was also very heavy. On the other hand, the USAAF had found it extremely reliable and simply "good enough" in air-to-air combat. This way the Browning became standard armament on American fighters - the P-51 Mustang, P-47 Thunderbolt or the F6F Hellcat, and more, with the same arrangement retained even for F-86 Sabre in post-war years.

When the same gun was being considered for the Spitfire, the Supermarine had at least two obvious choices, both of which would fit snugly within the universal structure of the C-wing then in production. 

• Placing four .50" Brownings in the C-wing cannon bays, at the same time removing the outer ".303 Brownings. This would give the Spitfire a complement of four .50-calibre guns, similar to P-51B Mustang.
   
• Placing a .50" Browning in the unused cannon bay of each wing plus remove the outer ".303 guns. This would give the Spitfire a complement of two 20 mm Hispanos and two .50" machine guns.

As is known, the latter combination prevailed to became widespread standard on Merlin-powered Spitfires during late 1944 and 1945. The main reason is probably because the RAF doctrine considered cannon, with its vastly superior hitting power, to be a necessary ingredient of fighter armament both in the air and against ground targets (compared with the Spitfire Mk. IXC, the P-51B was lightly armed indeed; its reputation in this area was probably saved by the fact that its opposition included mostly fighters).

On the other hand, replacing the rifle-calibre machine guns with a heavy machine gun brought back a degree of combat efficiency to machine gun fire. This was significant not least against the ground targets, which were to become important in operations of the 2nd Tactical Air Force in after the invasion of Europe.

Armourers inspect the M2 Brownings at the workbench

Because of the Browning's weight, the new installation was heavier than the previous arrangement, increasing the armament weight on the Spitfire from 235 to 276 kg. Mounting the M2 in the wing proved to be a relatively straightforward modification - the gun fit by a margin into one of the cannon bays of the "universal" C-type wing, with its barrel completely hidden within the wing's outline.

Thus the E wing was structurally identical to the Type C, differing only in armament installation. Castle Bromwich factory records do not even indicate "E" wing having entered production. Their wings, for the Merlin Spitfire, were all of the same universal type. It is therefore unclear how the "E" designation came about. It is possible that it was accepted later, when the introduction of low-back Mk. XVI led to placing the oxygen and compressed air bottles to be moved to vacant .303" Browning compartments in the wings. 

Diagram showing the layout of cannon and .5" M2 Browning installation in a Spitfire
[Crown Copyright]

Published at this site before, this picture shows the arrangement of the armament bay of the Spitfire LF Mk. IXE, with Hispano Mk. II cannon to the left and the M2 Browning to the right. Note how the entire bay was designed to accommodate two Hispanos. The Browning fits easily in its oversized space. Its ammunition bay (the top one) was simply "downsized"  from its initial dimensions through simple addition of a spacer along its forward edge.
[Martin Waligorski]

The first Spitfire to receive the new armament was MK197, an LF Mk. IX from Castle Bromwich production line. It was delivered to AAEE at Boscombe Down for armament trials on 11 February 1944.

Deliveries of production Mk. IXEs started the same month; it is believed that the initial production batch, completed until early April,  comprised 60 machines, which went to No. 66 and 504 Squadrons.

Simultaneously, modification instruction was issued to convert existing in-service Spitfires Mk. IXC with the new armament. This could not be carried out at the unit level as the modification included changes to the wing "plumbing" - presumably gun heating and compressed-air installations. Instead, Vickers Supermarine issued working parties to convert the aircraft.

The M2 Browning was mounted in the inner cannon bay, with its barrel hidden completely within the bulk of the wing. The prominent collar is not coupled with this installation, but is a feature carried over from the "C" wing - it provided a reinforced forward mount for the second Hispano cannon.
[Martin Waligorski]

The pace of the conversion work is very difficult to assess. Since the type designation of the converted aircraft remained unchanged in the records, it is rather impossible to establish how many Spitfires were converted by the time of D-Day. It is clear, however, that CBAF continued to roll out LF Mk. IXC aircraft with "old" armament during the same period.

An interesting curiosity is that No. 485 (New Zealand) Squadron converted several of their Spitfires LF Mk. IX of  to carry the .50 Brownings without resorting to the Supermarine instructions.

The first Spitfire LF Mk. XVI, MJ556, flew in December 1943 and carried "C"-type armament. The squadrons did not start receiving the new mark until the beginning of October 1944, when mass production of this variant commenced at the Castle Bromwich factory.  During winter 1944-1945, LF Mk. XVIE replaced the LF Mk. IX as the most common fighter type in the 2nd TAF on the Continent.

It is generally believed that all production examples of the Mk. XVI carried the "American" armament. This is, however, difficult to confirm with certainity. Confusingly, the designation "LF. XVIE" first appeared in CBAF records around May-June 1945. The first low-back Mk. XVI, SM410, left CBAF for trials on 30 March 1945, so it is possible that at the time of its introduction, the "E" suffix was intended to refer to the low-back Spitfire and/or the previously mentioned changes in wing plumbing incorporated in that variant. Perhaps we'll never know.

Upper an lower armament bay covers were replaced during the conversion, due to the cannon being moved to the outer bay and the different arrangement of ejection chutes.
[Martin Waligorski]