During World War II the Wehrmacht generally described small arms such as pistols and machine guns by the diameter of their bore in millimetres, the most common being 7.92 mm and 9mm. Larger calibre guns were usually categorised in centimetres. The practice in most countries was also to use millimetres, for example the American 75 mm M3. British tanks and anti-tank guns and some artillery pieces were differentiated by the weight of the projectile they fired, for example the 6-pounder and 17-pounder. However, some older British types retained their identification by the diameter of their bore in inches. ‘Calibre’ refers to the diameter of the barrel, usually omitting the rifling. We refer to a 7.92 mm-calibre MG34 medium machine gun. Small arms ammunition was frequently known by its calibre and cartridge length together, 7.92 mm x 57, or by attaching a common name to the calibre, such as Parabellum or ACP. These rules were not applied in any hard or fast manner and variations can be endless.
With the artillery the term calibre was used to describe the diameter of the bore of a gun’s barrel in relation to the length of the barrel. The Americans measure this length from the back of the chamber to the muzzle and the Germans measure from the back of the breech to the muzzle, therefore a gun of 56 calibres - often symbolized as L/56 - has a barrel 56 times the length of its nominal bore. In the case of the German KwK 36 8.8 cm L/56 gun, this means a total barrel length of 56 x 8.8 cm, or 4.93 m.
Rifle and machine-gun calibre ammunition came in many shapes, including incendiary and explosive types. The most common were the jacketed hard point (JHP), jacketed soft point (JSP), tracer and armour-piercing (AP). For the Tiger, three types of 8.8 cm ammunition were dispensed - armour-piercing, high explosive (HE) and high-explosive anti-tank (HEAT). The customary variety of armour-piercing was the Pzgr.39, a 10.2 kg armour-piercing capped, ballistic-capped round with an explosive centre which left the KwK36’s muzzle at a velocity of 773 m/sec.
AP rounds usually accounted for half of a Tiger’s ammunition supply, the rest taken up with Sprgr. HE rounds for use against soft-skinned vehicles. The hollow-charge Gr.39HL round, which was less productive at short range, was sometimes exchanged for some of the HE load despite being less accurate. The Pzgr.39 APCBC round was capable of piercing 120 mm of armour at an angle of 30 degrees within a range of 1000 m. The tungsten-cored Pzgr.40 round could easily pierce 170 mm of armour at short range and 110 mm at 2000 m, while the Gr.39HL round could penetrate 90 mm of armour up to 2000 m.