because it was still on their secret list.
Much more productive was the work on 37mm cannon, which produced several different types amid some confusion. The American Armament Corporation (AAC) marketed several weapons in this calibre. None of these was successful and suggestions by some sources that they were involved with the development of the Oldsmobile T9 (adopted into service in December 1939 as the 37mm M4) are unfounded.
The M4 was a Browning-type long-recoil gun (the barrel recoiled some 240mm with each shot) firing a 37 X 145R cartridge which was in effect an enlarged version of the old Pom-pom round. The muzzle velocity was much lower than that of the .50" Brownings which were usually fitted alongside it, which could cause significant aiming problems. At a range of 365m, the M4's projectiles dropped 137cm from a straight-line trajectory in comparison with 81cm for the .50"; at 730m the drops were 1,440cm and 867cm respectively. In addition, the times of flight varied. This was presumably one reason why most P-38s used the 20mm, which had ballistics much closer to those of the .50". Initial unreliability, caused mainly by ejection problems, did not help the M4's reputation, although this was eventually overcome.
The M4 was fitted to various experimental aircraft and to the few Lockheed P-38D Lightnings built, but saw most service in the Bell P-39 Airacobra and early versions of the P-63 Kingcobra. Initially, the ammunition capacity was only fifteen rounds but this was soon replaced by a thirty-round continuous-loop belt which forms a characteristic hoop around the breech, designed to fit the contour of the aircraft nose. The M10 was a version of the M4 adapted to use a disintegrating-link ammunition belt, together with some minor alterations to raise the rate of fire, and was fitted to later versions of the P-63 (A-9 onwards), in which the ammunition capacity was increased to fifty-eight rounds. Many P-39s and most P-63s saw action in Soviet hands. A combination of .50" HMGs and 37mm cannon (presumably MIOs) was initially proposed as defensive armament for the giant B-36 bomber, but this was changed to a uniform 20mm battery at the design stage.
Some mystery surrounds the use of the third of
Magazine ( M6) for the 37 mm M4 gun. The 30-round endless belt magazine was shaped to fit the nose contours of the P-39 Airacobra. It measured 67 x 46 X 26cm and weighed 16kg (unloaded)
(Courtesy: MoD Pattern Room)
the US 37mm aircraft cannon, the M9, which was a version of the armys M1 AA gun modified to use a disintegrating-link belt. Despite being officially adopted, it only appears to have been fitted in test installations, in aircraft such as the single example of the experimental P-63D (in which forty-eight rounds were carried). Data concerning the M4 and M9 are sometimes confused in aircraft publications, perhaps because the M4 started out life being known as the T9. Some sources suggest that the M9 did see Soviet service in versions of the P-39 and/or P-63 but it has not proved possible to establish this and there is no reference to it in Soviet sources, which do give details of their use of the M4. The only confirmed service use was its installation, towards the end of the war, in some USN PT boats. Some aircraft which failed to reach production namely the XP-58, XP-67 and XP-72, were reportedly intended to be fitted with 37mm guns, but it is not clear which type. Some effort was put into developing slimmer and faster-firing guns such as the T37, but these remained prototypes.
The M9 would undoubtedly have made a formidable 'tank buster', as some high-velocity AP loadings of the 37 X 223SR cartridge were developed with about 50% more muzzle energy than the equivalent standard BK 3.7 ammunition used by Germany. The M80 shot had a claimed penetration of homogeneous plate of 79mm/450m/70°.
The Americans did field some 75mm ground-attack cannon. The M4 was a modified medium-velocity army gun (the later AN-M5 being a lightweight version) and these were carried by some versions of the North American B-25 Mitchell bomber. They were manually loaded, but proved effective against shipping. A version with an automatic feed mechanism to achieve 30 rpm, the M10, was developed by the end of the war but did not see action, the priority having switched to airborne rockets.
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