Soon after the German invasion of the USSR in 1941, Wehrmacht units encountered new Soviet tanks: the medium T-34 and the heavy KV. The thick sloped armor of these vehicles gave them a good degree of protection against German anti-tank weapons. The situation eventually led to requests for more powerful guns that would be able to destroy the aforementioned tanks from long range. Germany already had a suitable design, the 7.5 cm Pak 40, entering production in late 1941, but the first pieces were not delivered until 1942. Until enough of those would be manufactured, expedient solutions were required.
In the early stage of the war, Germans captured a large number (up to 1,300) of Soviet 76-mm divisional guns model 1936 (F-22). Developed with anti-aircraft abilities in mind, the Soviet gun had powerful ballistics; it was also originally intended to use more powerful cartridge than the one eventually adopted. However the design had some shortcomings in the anti-tank role. The shield was too high, the two man laying was inconvenient and the sighting system was more suitable for the F-22 original divisional field gun role. Using considerable thrift, the German engineers were able to quickly modify the F-22, which by that time had been adopted in original form as the FK296(r) by the Wehrmacht. In late 1941 German engineers developed a modernization program. The initial modifications that brought the guns to FK36(r) standard included:
- removing the top section of the shield and using the armour off-cuts to superimpose over the lower section of the shield. These were held in place using the standard Pak38 shield pintles.
- re-orienting the traverse gear box and handwheel shaft linkages so as to mount the traverse handwheel on the left side of the gun next to the sight.. As the new rod linkage went through a gap in the recoil cradle's elevation arc, the maximum elevation angle was limited to 18 degrees.
- replacement of the Russian sight with a Pak 38 style anti-tank sighting block that could mount the standard ZF3x8 sighting telescope or an emergency fold out iron sight. Apart from having a clinometer plane on the sight mount, there was no provision for indirect sighting.
The first of these converted F-22s retained the original Russian ammunition (confirmed by measuring the chamber length of 15.2 inches or 385 mm, and were still designated FK296(r) on the sight's range drum. These early anti-tank conversions are discernable as they have not been fitted with a muzzle brake. These intermediate guns had various designations, but appear to have been referred to mainly as "FK36(r)", despite their dedicated anti-tank role seeming to warrant the designation "Pak" rather than "FK". The conversion work was performed by HANOMAG with sight blocks made by Kerner & Co in 1942 (ggn42).
Later up-grades were designated as the Pak36(r), and had:
- Rechambering for the more powerful Pak40 cartridge. The German cartridge was nearly twice as long as the Soviet one (715 mm vs 385.3 mm) and wider (100 mm vs 90 mm), resulting in 2.4 times larger propellant load.
- Recoil mechanism adjustments to accommodate the new recoil characteristics.
Read more about this topic: 7.62 Cm Pak 36(r)
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