7.5 Cm Pak 40 - Performance

Performance

The weapon was effective against almost every Allied tank until the end of the war. The Pak 40 was much heavier than the 5 cm 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 Russia where it was needed to combat the newest Soviet tanks. It was designed to fire the same low-capacity APCBC, HE and HL projectiles which had been standardized for use in the long barreled Kampfwagenkanone KwK 40 tank-mounted guns. 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.

The main differences amongst the rounds fired by 75 mm German guns were in the length and shape of the cartridge cases for the Pak 40. The 7.5 cm KwK (tank) fixed cartridge case is twice the length of the 7.5 cm KwK 37 (short barrelled 75 mm), and the 7.5 cm Pak 40 cartridge is a third longer than the 7.5 cm KwK 40.

The longer cartridge case allowed a larger charge to be used and a higher velocity for the armour-piercing capped ballistic cap round to be achieved. The muzzle velocity was about 790 m/s (2,600 ft/s) as opposed to 750 m/s (2,500 ft/s) for the KwK 40 L/43. This velocity was available for about one year after the weapon's introduction. Around the same time, the Panzer IVs 7.5 cm KwK 40 L/43 gun and the nearly identical Sturmkanone (StuK) 40 L/43 began to be upgraded with barrels that were 48 calibers long (L/48), which remained the standard for them until the end of the war.

In the field, an alarming number of L/48 cartridge cases carrying the hotter charge failed to be ejected properly from the weapon's semi-automatic breech, even on the first shot (in vehicles). Rather than re-engineer the case, German Ordnance reduced the charge loading until the problem went away. The new charge brought the muzzle velocity down to 750 m/s (2,500 ft/s), about 10 m/s higher than the original L/43 version of the weapon. Considering the average variability in large round velocities from a given gun, this is virtually negligible in effect. The first formal documentation of this decision appears on May 15, 1943 ("7.5cm Sturmkanone 40 Beschreibung") which details a side by side comparison of the L/43 and the L/48 weapons. The synopsis provided indicates very little difference in the guns, meaning the upgrade had little if any benefit.

All further official presentations of the KwK 40 L/48 ( "Oberkommando des Heeres, Durchschlagsleistungen panzerbrechender Waffen") indicate a muzzle velocity of 750 m/s for the gun. As for the Pak 40, the desire for commonality again appears to have prevailed since the APCBC charge was reduced to 750 m/s, even though case ejection failures apparently were never a problem in the Pak version of the gun.

For unknown reasons some 75 mm APCBC cartridges appear to have produced with a charge which gave a muzzle velocity of about 770 m/s (2,500 ft/s). The first documented firing by the U.S. 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.