Aviation History

The Me 410 and the Colossal Five-Centimetre BK 5 Cannon: the Story of ‘The Backbone of Reich Defence’

When, in November 1943, Hitler watched a demonstration of an Me 410 equipped with a BK 5 cannon, he enthused that it was the ‘backbone of Reich air defence’.

Intended as a progressive development of the twin-engined Bf 110 Zerstorer, the Me 210 first took to the air in September 1939. Despite the shortcomings associated with the Bf 110, the Luftwaffe pressed ahead with the development of the Me 210 and its successor, the Me 410. Actually, with enhancements made to the fuselage and wings, as well as providing extra power, the Me 210 became the Me 410 in late 1942.

Both aircraft were deployed as heavy fighters, fighter-bombers, reconnaissance platforms and interceptors. The Me 410 was fitted with 30 mm cannon, 21 cm underwing mortars and the colossal Rheinmetall five-centimetre BK (Bordkanone) 5 cannon that was intended to pack a punch against the USAAF’s four-engined bombers.

As told by Robert Forsyth in his book Me 210/410 Zerstorer Units, the Luftwaffe Technisches Amt had issued a requirement for a gun with a muzzle velocity of at least 600 m per second and a rate of fire of 300 rounds per minute, the concept being to have a weapon capable of inflicting almost certain damage on enemy bombers, but without the attacking aircraft coming within range of the enemy’s defensive fire. However, the dichotomy was that while there was a requirement for accuracy and extremely low dispersion, due to the heavy-calibre ammunition required, it would be impossible to achieve a high rate of fire. Every shot would have to count. The two aircraft selected to trial such a weapon were the Me 410 and Ju 88.

The idea of using a five-centimetre cannon in the Me 410 was first proposed by officers on the staff of the General der Jagdflieger to E.Kdo 25 in July 1943. The weapon they envisaged being used was an adaptation of the Wehrmacht’s five-centimetre KwK (Kampficagenkanone) 39 tank gun. Using the Revi 12C/D or Zielfernrohr ZFR 4A telescopic sight, a hit with one five-centimetre round was thought to be enough to bring down a bomber. However, effective employment of the ZFR 4A required considerable gunnery and flying skill, or much practice, as well as the ability to stay in a stable firing position for a long period of time.

The alterations required to fit the big gun into an aircraft such as the Me 410 were thought acceptable, given that this would avoid the development of a new weapon. Furthermore, the incorporation of a magazine would make the cannon fully automatic, and the gun, the mounting and magazine-feed mechanism could be assembled as one unit to make it easily interchangeable. Loading was performed electro-pneumatically, whilst the ammunition was fired by an electric primer. Recoil was taken by two hydro-pneumatic cylinders mounted above the gun cradle.

 Basic side-by-side comparison of the Me 210 and Me 410 wing planforms

In the Me 410, the rear part of the gun was fixed to the main spar via an adjusting mechanism on a plate, and the bomb-bay doors were removed and replaced by a ventral fairing. The installation of the gun proved so satisfactory that the outline of the ventral fairing needed only to be increased by some 100 mm at its deepest point as compared with the original doors. A cartridge ejection opening with an internally-mounted chute was cut into the rear of the panel. By using a perforated muzzle-brake, muzzle blast was so distributed over the airframe that no damage was caused, and therefore no reinforcement necessary. The only problem was the challenge of keeping the gun heated at altitude, and thus heating was provided from the crew compartment.

The gun was hoisted into an Me 410 by means of a large, mobile hydraulic jack known as a Steinbock. The muzzle end of the barrel was fastened by means of a rope to the arm of the Steinbock, which was then pumped up until the rear end of the mounting was inclining upwards. The Steinbock, together with the gun, was then run in under the fuselage and the weapon raised until the clamp of the setting mechanism was in line with the plate fixed to the main spar. Two locking bolts were then screwed in. A gimbal ring, a pair of lateral bracing tubes and a suspension stirrup were used to secure the mounting of the gun.

By late October 1943, it seems Galland’s office had revised its opinion of the BK 5, and it now felt that a rapid-fire weapon such as the MK 103 would be better. Nevertheless, when, in November Hitler watched a demonstration of an Me 410A-1 equipped with a BK 5 cannon, he enthused that it was the ‘backbone of the home air defence’. The Fuhrer demanded that the Me 410 be committed to Reich defence, and that two Gruppen or a Geschwader be equipped with the aircraft/ weapon combination.

Goring had to pluck up courage and advise Hitler that only two or three aircraft could be equipped with the BK 5, since there were no further cannon available. His advice having fallen on stony ground, and in view of the fact that the bulk of Me 410s had been assigned to bomber Gruppen for operations over England, the Reichsmarschall instructed Generalfeldmarschall Mitch on Jan. 12, 1944 to set about equipping two Gruppen with 45 cannon-fitted aircraft each. Milch wrote to Goring in response two days later;

`The first experimental aircraft have been equipped with a 50 mm gun. Since this gun can no longer be delivered, a conversion for a new 50 mm gun had to be carried out within a very short period of time. This new gun is installed in a different manner. The first delivery of ten guns was to be made in December. This was not possible since the magazine feeding device which we constructed suffered stoppages due to broken belt links, whereby the cartridge was hitched on the belt conveyer table. These defects have now been eliminated. There have been difficulties experienced with the high-explosive shell we developed on account of casting defects in the casings and also in respect to its dispersion because of the rotating bands. Alterations are being carried out in the plants. The results have still to be tested.’

Once initial investigation into the BK 5-equipped Me 410 V2 had been carried out at Tarnewitz, adaptation of the gun for aerial use was undertaken by Deutsche Luft Hansa at Berlin-Staaken. After just over three months of testing, during which various belt-feed and jamming malfunctions had been ironed out, the resulting weapon was installed in aircraft of II./ZG 26 as the Me 410A-1/U4 from early February 1944.

The Gruppe moved from Hildesheim to Oberpfaffenhofen to re-equip, and by Feb. 8, 5. Staffel had 12 cannon-equipped aircraft. Trial operations duly commenced over southern Germany and Austria, despite the fact that the weapon was still experiencing electrical problems, small switches were found to break easily and ammunition belts continued to fall apart. Also, the BK 5’s recoil and feed mechanisms were unable to cope with the g-forces of aerial combat, and it was rare for more than one shell to be fired without the weapon jamming. These issues were mitigated to some extent by the introduction of an improvised clearing device.

Generally, however, the employment of large-calibre guns, forced onto the units by Goring and the high command, proved a fallacy. It would have been better to have used unguided rockets then under development.

In October 1943, the daylight battle over the Reich reached its zenith, forcing the USAAF to accept that unescorted, deep penetration formations could not adequately protect themselves. However, although the losses incurred during such missions had reached unacceptable levels, they nevertheless forced the Luftwaffe into the air to fight, and in doing so inflicted attrition on a scale from which the Germans would find it difficult to recover.

On Oct. 10, the Kommodore of ZG 26, Major Karl Boehm-Tettelbach, led his Bf 110s, together with Me 410s of III./ZG 1, from Hesepe and Osnabruck against B-17s of the 3rd Bomb Division during an attack on the marshalling yards at Munster. The 3rd’s B-17 s, left without any escort due to bad weather over England, had already been mauled by single-engined fighters when the 14th Bombardment Wing was particularly badly hit near Munster by the Zerstorer in a mass attack from the rear. Indeed, it was to be their single most successful day, with the Bf 110s of ZG 26 claiming 14 B-17s destroyed. Conversely, and somewhat inexplicably, the Me 410s of III./ZG 1 returned with no laurels.

Nor did missions by I./KG 51 in early October achieve much. On the 14th 229 of 291 B-17s despatched managed to reach Schweinfurt in a return to the aircraft industry targets that had proved so costly to the USAAF in August. Under direction of the Jagdfliegerfuhrer Ostmark, the Me 410s of I./KG 51 took off from Horsching and headed west until they made visual contact near Schweinfurt, where the bombers had already unloaded their ordnance on the city’s ball-bearing factory. The Messerschmitts chased the bombers, fired their mortars and followed them in towards the enemy formation. Once again, the Me 410s found themselves trapped by the bombers’ highly effective defensive fire, and the Gruppe was lucky to escape without loss — but neither did it make any claims. It proved to be the unit’s last such mission before being reassigned for missions over England.

A Messerschmitt Me 410 with a BK 5 cannon peels off from attacking a 388th Bomb Group B-17 over Europe during the USAAF campaign against Germany, 1943

Me 210/410 Zerstorer Units is published by Osprey Publishing and is available to order here.

Photo credit: U.S. Air Force, The PIPE and Mike Freer via Wikipedia

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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