7.5 cm Pak 40

The 7.5 cm Pak 40 ("7,5 cm Panzerabwehrkanone 40 -" "7.5cm armour defence cannon [19]40") was a German 75 millimetre anti-tank gun of the Second World War.
The gun was developed in 1939–1941 and entered service in 1942. With 23,303 examples produced, the Pak 40 formed the backbone of German anti-tank guns for the later part of World War II, mostly in towed form, but also on a number of self propelled artillery such as the "Marder" series of "Panzerjäger"
A modified version of the gun designed specifically for vehicle-mounting was the 7.5 cm KwK 40, which differed primarily in using more compact ammunition, thereby allowing more rounds to be carried inside the vehicles. The KwK 40 armed many of the German mid-war tank and tank destroyer designs such as the Panzer IV, replacing the Pak 40 in the latter role.
The Pak 40 may be referred to as the 7.5/L46, referring to its calibre and the barrel's length in calibres. There were two versions of the KwK 40, which would be referred to as the 7.5/L43 or 7.5/L48.
Development.
Development of the Pak 40 began after reports of new Soviet tank designs began to reach Berlin in 1939. The German army was equipped with the 3.7cm Pak 36 at the time. A replacement 5 cm Pak 38 was still in testing at this point, but it appeared it would not be powerful enough to deal with these newer designs. Contracts were placed with Krupp and Rheinmetall to develop what was essentially a 7.5 cm version of the Pak 38. However, while the Pak 38 made extensive use of light alloys to reduce overall gun weight, these were now earmarked for aircraft production to supply the "Luftwaffe". As a result, the Pak 40 used steel throughout its construction and was proportionally heavier than the 5 cm model. To simplify production, the Pak 38's curved gun shield was replaced by one using three flat plates. A version called the 7.5 cm FK 7M59 was proposed towards the end of the war to fill a dual-purpose role of field gun and anti-tank gun. The carriage was modified to provide +35° of elevation which increased maximum range to . Another dual purpose variant was the 7.5 cm FK 7M85 which used the gun and recoil system of the Pak 40 on the carriage of the 10 cm le FH 18/40.
The project was initially given low priority, but following the invasion of the USSR in 1941 and the appearance of heavily armoured Soviet tanks such as the T-34 and KV-1, it was given an increased priority. The first pre-production guns were delivered in November 1941. In April 1942, the "Wehrmacht" had 44 guns in service; by 1943, the Pak 40 formed the bulk of German anti-tank artillery.
Operational use.
The Pak 40 was the standard German anti-tank gun until the end of the war, and was supplied by Germany to its allies. Some captured guns were used by the Soviet Red Army. 
23,303 Pak 40s were produced, and about 3,500 more were used to arm tank destroyers. The unit manufacturing cost amounted to 2,200 man-hours at 12,000 RM
A lighter version of the Pak 40 was used as the "BK 7,5" automatic "weapon system" in the Henschel Hs 129B-3 and the Junkers Ju 88P-1 ground attack aircraft. This version of the Pak 40 was the heaviest of the "Bordkanone" series of heavy calibre aircraft guns, incorporating a twelve-round magazine. This version of the gun was also intended as a production fitment for a possible He 177A-3/R5 heavy bomber adaptation late in 1942, originally prototyped in the field with BK 5 cannons, themselves adapted from the 5 cm KwK 39 tank gun from the Panzer III.
During the second half of World War II, some Romanian anti-tank platoons each had three Pak 40 guns. These were used interchangeably with Romania's own 75 mm Reșița Model 1943 anti-tank gun.
Performance.
The weapon was effective against almost every Allied tank until the end of the war, only struggling to penetrate heavier vehicles like the Russian IS tanks, the American M4A3E2 Sherman 'Jumbo' assault tank and M26 Pershing, and later variants of the British Churchill tank. The Pak 40 was much heavier than the Pak 38; its decreased mobility meant that it was difficult or even impossible to move without an artillery tractor on boggy ground.
The Pak 40 was first used in the USSR where it was needed to combat the newest Soviet tanks. It was designed to fire the same low-capacity APCBC, HE and HL projectiles that had been standardized for use in the long barrelled "Kampfwagenkanone" KwK 40 tank-mounted guns of the mid-war and later marks of the Panzer IV medium tank. In addition, there was an APCR shot ("Panzergranate" 40) for the Pak 40, a munition which - reliant on supplies of tungsten - eventually became very scarce. According to the German "Panzertruppen" news journal, 5,000 APCR rounds were expected in Dec. 1942 as replenishment for the Winter offensive.
The main differences amongst the rounds fired by 75 mm German guns were in the length and shape of the cartridge cases as well as the primers used. The 7.5 cm KwK 40 (75x495mm) used in tanks had a fixed cartridge case twice the length of that used by the 7.5 cm KwK 37, the short barrelled 75 mm used on earlier tanks, and the 7.5 cm Pak 40 cartridge was a third longer than that used by the KwK 40. The Pak 40 used a percussion primer, while the vehicle mounted 75 mm guns used electrical primers. Other than minor differences with the projectiles' driving bands, all German 75 mm guns used the same 75mm projectiles.
The longer cartridge case of the Pak 40 allowed a larger charge to be used and a higher velocity for the PzGr 39 armour-piercing capped ballistic cap round to be achieved. The muzzle velocity was about as opposed to for the KwK 40 L/43 and for the L/48. The only 75mm fighting vehicle gun in general use by Germany that possessed a longer barrel than the Pak 40, the 7.5 cm KwK 42 on the Panther tank, could achieve a higher muzzle velocity of 935 m/s (3,070 ft/s) using more propellant in a larger cartridge fixed to it for the KwK 42's use.
For unknown reasons, some 75 mm APCBC cartridges appear to have been produced with a charge that gave a muzzle velocity of about . The first documented firing by the US of a Pak 40 recorded an average muzzle velocity of 776 m/s for its nine most instrumented firings. Probably because of these results, period intelligence publications ("Handbook on German Military Forces") gave about 770 m/s as the Pak 40 APCBC muzzle velocity. Post war publications corrected this.
German sources differ; the Official Firing Table document for the 75 mm KwK 40, StuK 40 and the Pak 40 dated October, 1943, gives 770 m/s on one of the APCBC tables.
Ammunition.
An armour-piercing, capped, ballistic cap (APCBC) projectile with explosive filler and tracer.
An armour-piercing, composite rigid (APCR) projectile with a sub-calibre tungsten core.
A high-explosive anti-tank (HEAT) projectile with a shaped charge.
Post-war use.
After the war, the Pak 40 remained in service in several European armies, including Albania, Bulgaria, Czechoslovakia, Finland, Norway, Hungary and Romania. In 1955, USSR supplied 33 captured guns to Austria. They were kept in service into the 1960s. North Vietnam also received some during the Vietnam War. Six ex-Portuguese Army Pak 40s divided into two artillery detachments were stationed in Dili during the Indonesian invasion of East Timor in 1975.
Survivors.
Pak 40s are or have been held in several military museums, outside museums or free entrance open-air fields: