BK 37

The Bordkanone BK 3,7 ((on-)board cannon 37) was a 37mm anti-tank/bomber autocannon based on the earlier 37 mm Flak 18 made by Rheinmetall. It was mounted on World War II Luftwaffe aircraft such as the anti-tank or bomber-destroyer versions of the Junkers Ju 87D-3 and G-2, Henschel Hs 129B-2/R3, Messerschmitt Bf 110G-2/R1-3, and others. The cannon could be attached under the wings or fuselage of the aircraft as a self-contained gun pod with a 12-round magazine. It fired APCR (Tungsten hard core) ammunition or high explosive shells in 37x263B mm caliber at 160 rounds per minute.

Service history
BK-37 equipped ground attack aircraft were developed for use in the anti-tank role on the Eastern Front in a somewhat desperate effort to blunt the massive numerical superiority of the Soviet T-34 as the war turned against Germany. The concept was rather rudimentary, suffered from various issues (primarily poor accuracy, severe weight penalty making the craft vulnerable to fighters, and a low ammunition capacity), but could be extremely effective when operated by a sufficiently skilled and practiced ground-attack pilot (Hans-Ulrich Rudel in his BK-37 armed Stuka being the ultimate example).

The heavy caliber autocannon-armed series of Ju 88P twin-engined attack aircraft series used twin BK 3,7 cannon, mounted side-by-side in a conformal ventral fuselage gun pod, in its Ju 88P-2 and P-3 versions. The P-3 version only differed through the addition of extra defensive armor. As with other examples of the P-series, the Ju 88P-2 and P-3 were perceived as failures in both anti-tank and bomber destroyer role.

In contrast to the previous method (bombs delivered by dive bombing), when the BK-37 was employed in a top attack profile against the especially thin upper turret and engine compartment armor of a tank, kills could be achieved with a relatively light and cheap armor piercing projectile that could be carried in much greater quantities than bombs, but would be insufficient to penetrate if fired horizontally from the ground in the normal method. This was the first known implementation of a medium airborne cannon in the top down anti-tank role, and was the direct inspiration for later designs such as the 30mm GAU-8/A Avenger equipping the A-10 Thunderbolt II. Although revolutionary, the concept was implemented too late and in too few numbers to appreciably impact the course of the war for Germany.

One of the two surviving Junkers Ju 87s is a G-2 model displayed at the Royal Air Force Museum London. The aircraft's wings have the attachment points for BK 37 gun pods, but it is not displayed with them fitted.

Specifications

 * Type: single-barrel automatic cannon
 * Caliber: 37 mm (1.46 in)
 * Cartridge: 37 mm x 263
 * Operation: Short Recoil
 * Length: 3.63 m (11 ft 11 in)
 * Weight: 295 kg (650 lb)
 * Rate of fire: 160 rpm
 * Muzzle velocity: 1,170 to 780 m/s (3,836 to 2,557 ft/s)
 * Projectile weight: APCR 380 g, HE 640 g, AT 685 g