Flying Guns: Ammunition

Ammunition Types

The following types of ammunition were common on aircraft guns for for anti-aircraft use, and to some degree still are, even if the importance of aircraft guns has strongly decreased after the introductory of guided missiles.

AP. Armour-piercing rounds may be simple, solid projectiles. More often they have a sleeve of soft, light metal wrapped around a hard core, for example hardened steel or tungsten. The hard core is the penetrator. The sleeve is kept light to reduce the total weight of the round, so that the projectile is given a higher velocity. Its softness reduces the probability that the projectile will glance off armour. And in small-calibre weapons, it also grips the rifling of the barrel. Post-WWII developments such as discarding-sabot AP ammunition (APDS) are not commonly used against aircraft, but may be carried for use against ground targets.

HE. High Explosive rounds were traditionally made by boring out the core of a solid projectile, then filling it with explosive. The German Minengeschoß rounds introduced a different manufacturing technique: A thin shell was drawn by stretching and forming a metal block in steps, in the same way as a cartridge case is drawn. This resulted in a much larger explosive capacity for the same calibre, and became widespread after the war. It is commonly assumed that HE ammunition is really effective only in calibres of 20mm or larger, but it was also made for 12.7mm and even rifle-calibre weapons.

I. Incendiary rounds were first developed in small calibres during WWI. They were similar to high-explosive rounds, but the bullet was filled with incendiary rather than explosive material. Early on, the material was often phosphorus, ignited by the actual firing of the round. Later fused projectiles, which ignited only when hitting the target, also appeared. Pure incendiary ammunition was often replaced by high-explosive ammunition with an incendiary compound mixed in, labeled HE/I or HEI.

SAP/I or SAP/HE. Semi-Armour Piercing rounds are similar to the traditional designs for HE and I rounds, but the hollow outer shell is stronger and made of hardened material, so that some armour-penetrating capacity is retained. Optimists may call such ammunition AP/I or AP/HE. Modern fillings can combine both effects, so that SAPHEI ammunition is created. 

Tracer rounds have some material in the base of the projectile, which burns during flight and indicates the trajectory. For use at night “glowing” ammunition, which gives a fainter light, was developed, to avoid disrupting the night vision of the pilot or gunner. The disadvantage, especially in rifle-calibre ammunition, is that the tracer rounds have a different trajectory from the other ones, because of their lighter weight, but also because they eject gas from their tail. In addition, the high visibility of tracer alerts the target, but it may also have a deterrent effect.

Self-destruct systems can be simple chemical systems, which take a preset time to burn, or intricate mechanical fuses. They are designed to avoid “collateral” damage, and may also be used for training.

A very unusual type of ammunition was this T-44 .30 frangible round. The projectiles were made from a mixture of lead and bakelite, and American trainee gunners fired them at specially armored Bell RP-63 Kingcobras. Despite the RP-63’s armor, muzzle velocity had to be kept down to 415m/sec, and the recoil-operated Browning .30 M2 guns needed some gas assistance to function. Gunsights and aircraft speeds were chosen to give trainees the correct “feeling” of the .50 that was the standard USAAF armament of the time. [Operation Pinball, by Yvan Hickman]

Ammunition had to be fed into a gun, and it had to be stored in the aircraft. The two main alternatives are the drum and the belt (clips are hardly an option for aircraft guns). The drum is the simplest solution: It contains loose rounds, which are often fed into the gun by means of a circular spring. However, drums often have inconvenient shapes for installation in aircraft, they are bulky, and they contain a fixed, often small number of rounds. A much better solution was to link the rounds together to a belt, that could be stored in a box or tray with a convenient shape. Often the belt is of disintegrating type: After removal of the rounds, it falls apart in links which can be stored or dumped overboard. The belt can be made as long as the feeding mechanism can pull. However, the design of a belt feed mechanism is more complicated than that of a drum, and suitable feed mechanisms took a long time to perfect. During WWI most cannon had drums at the time of their first application, but at the end of the war almost everyone had switched to belts.

German Ammunition Belt Compositions

For German Fighters

These are the belt compositions for fighters, used against air targets, as given given in a German manual, published in in 1944, the so-called Merkbuch über die Munition. Note that these were more or less advisory: Local commanders were encouraged to determine the armament mix that suited them.

7.92 mm (MG 17)

  • 5 SmK-v
  • 4 PmK-v
  • 1 B-Patrone-v

SmK ammunition was AP with a hard steel core and a lead sleeve. The probable explanation of the acronym is Spitzgeschoss mit Kern, pointed ball with core. PmK also had a steel core, but the core was surrounded by phosphorus, which ignited when the round was fired. Finally the B-Geschoß was a Beobachtungs or observation round: It had a small HE charge and some incendiary material, and exploded on contact with the target. In this way the pilot was able to verify that he was hitting the target. During the Battle of Britain, the British used the Dixon-De Wilde round for similar purposes, and pilots generally felt that this was extremely useful.

13 mm (MG 131)

  • 1 Panzergranatpatrone L’spur o. Zerl
  • 2 Brandsprenggranatpatronen L’spur o. Zerl

The 13mm Panzergranatpatrone was a solid AP round. The Brandsprenggranatpatrone was a conventional HE/I round, a bored-out projectile filled with an explosive mixture. German armourers were warned that the first round fired had to be an AP round: The cap over the muzzle had to be destroyed first, and there was the possibility that the HE/I round would go off when it hit this. Note that for both rounds, tracer was chosen (L’spur, or Leuchtspur) but that there was no selfdestruction (o. Zerl, or ohne Zerlegerung).

15 mm (MG 151)

  • 4 Brandsprenggranatpatronen L’spur m. Zerl
  • 1 Panzergranatpatrone L’spur o. Zerl

Rather similar to the 13mm, except that the HE/I rounds now do have self-destruction mechanisms. It was common to use a combined self-destruction fuse and tracer: The projectile exploded when the tracer was burnt out. On some projectiles, special self-destruction fuses were used. They were set to 3 seconds, except before April 1941 when they were set to 1.7 seconds.

The MG 151 was a high-velocity weapon, and for ground attack missions Hartkernmunition, AP with a tungsten core, was loaded.

20 mm (MG-FF, MG 151/20)

  • 2 Minengeschoß m. Zerl.
  • 2 Brandsprenggranatpatronen L’spur m. Zerl 
    oder Brandgranatpatronen
  • 1 Panzersprenggranatpatrone o. Zerl 
    oder Panzerbrandgranatpatrone (Phospor) o. Zerl.

Here the Minengeschoß appears for the first time. A version of the 20mm M-Geschoß with tracer did not exist, so tracer was used on conventional HE/I (Brandsprenggranatpatrone) or pure incendiary (Brandgranatpatrone) rounds. The latter was apparently a new development in 1944, intended to replace the less effective HE/I. The fifth round was a semi-AP projectile, explosive or incendiary. Apparently the main reason this was used instead of a solid AP round was that a solid projectile would have been too heavy and have a too different trajectory.

It was recommended that more AP or semi-AP ammunition would be loaded when the probable targets were well-armoured attack aircraft such as the Il-2. On the other hand, against the four-engined bombers of the RAF and USAAF the high explosive types were more effective.

30 mm low-velocity (MK 108)

  • Minengeschoß 108 El o. Zerl. 

Only the Minengeschoß was fired by the MK 108, also in versions with day or night tracer. The ammunition was not interchangeable with that of the much more powerful MK 101 and MK 103, hence the addition 108. The letters El probably indicate the presence of Elektron, an incendiary compound, in the projectiles. Surprisingly, self-destruction fuses were not used, although German fighters were operating over the home country at this time in the war. Maybe it was felt that this reduced the effective range too much, or it was just an economy measure.

30 mm high-velocity (MK 101, MK 103)

  • 1 Sprenggranatpatrone L’Spur o. Zerl
  • 1 Minengeschoß L’Spur o. Zerl
  • 1 Panzersprenggranatpatrone L’Spur o. Zerl 
    oder Panzerbrandsprenggranatpatrone L’Spur o. Zerl

The MK 103 was a high-velocity weapon with a much better armour penetration than the MK 108. Hence the addition of the older type of HE round and semi-AP ammunition to the mix. The exception were the nightfighters, which used only the Minengeschoß with a glowing trace (Gl’spur).

For anti-tank missions, Hartkernmunition with tungsten cores was used, but it would be wasteful to use this scarce ammunition against aircraft. 

For German Bombers

For bomber defensive guns of 7.92mm and 13mm calibre, the following combinations were recommended:

7.92 mm (MG 15, MG 17, MG 81)

  • 2 SmK
  • 2 SmK L’spur oder SmK Gl’spur
  • 2 PmK
  • 2 SmK
  • 2 SmK L’spur oder SmK Gl’spur
  • 1 PmK
  • 1 B-Geschoß

The main difference with the ammunitions mix for fighters is in the use of tracer, avoided for fighters except to mark the end of the belt. On the other hand, only one in twelve rounds is the B-Geschoß.

13 mm (MG 131)

  • 1 Panzergranatpatrone L’spur o. Zerl
  • 1 Brandsprenggranatpatrone o. Zerl
  • 1 Sprenggranatpatrone L’Spur Üb m. Zerl

This load is a mixture of AP and HE/I with training ammunition (Übung) with self-destruct fuses! This was used in the MG 131 because it detonated after about 700m, and the flashes had a deterrent effect on attacking fighters. The relatively generous use of tracer and phosphorus ammunition in the MG 17 probably had a similar background.

Next: Ballistics