Available for all weapons:
Standard: Moderately effective at all distances. No chance of setting opponent on fire.
Armor-Piercing: Higher damage at greater distances. No chance of setting opponent on fire.
Incendiary: Lower damage at greater distances. 20% chance to set opponent on fire.
Armor-Piercing-Incendiary: Slightly more damage at greater distances. 15% chance to set opponent on fire.
Only available for autocannons (20 mm+):
Solid Core Armor-Piercing: Significantly higher damage at greater distances. No chance to set opponent on fire. Germany only.
High Explosive-Incendiary: Lower damage at short distances. Significantly more damage at greater distances. 30% chance to set opponent on fire.
High Explosive: Lower damage at short distances. Even higher damage at greater distances. 10% chance to set opponent on fire.
In general the game assigns higher armor penetration values to higher calibers. Shooting a heavily armored target with a low caliber will result in very low damage or possibly no damage at all. It's no where near as extreme as in World of Tanks, but it's still an important consideration. There are a few points to be made in regards to this statement:
- What the game refers to as "caliber" is the diameter of the barrel the cartridge is fired from. It is not caliber in the Imperial measurement system sense. For simplicity, the game lists ammunition for all nations in millimeters, even though they did not historically use the metric system. The one exception to this is the .303 British.
- Different planes and ground targets in the game have varying amounts of armor. It isn't listed anywhere, so there's no way to quantitatively compare one plane to another. In general, heavy fighters and attack planes have the strongest armor, in addition to having the most hit points. Armor also generally increases along with tiers. Ground targets are marked as light, medium, or heavy.
- We also don't know if the game looks at muzzle velocity in regards to armor penetration. It does list muzzle velocity, but it may just be there for bullet travel calculations.
- We do not know if the game considers cartridge length (For example, does the game consider the American and Soviet 7.62 mm the same, or does it know that they're different lengths, with the longer one being more powerful?). Play testing leans towards the game considering them the same. Unless the Devs tell us though , there's no way to be certain.
In any case, we know that beyond the first two tiers, low caliber weapons such as 7.62 mm quickly become incapable of doing much damage. On some planes, which have larger caliber weapons and a few small caliber weapons, the player can use incendiary ammunition on the low caliber weapons. Since they don't do much damage anyway, losing some damage won't matter, but the chance of lighting the target on fire remains, regardless of armor.
Full Names of Machine Gun Ammo
7.62x51 mm - USA. Historically known as ".30 cal." Sometimes referred to as "7.62 mm NATO" in modern times. Seems to be the weakest in the game.
7.62x54 mm R - USSR. Originally developed for the Maxim machine gun. Hard to see any difference in from the 7.62 used by the USA in the game. However, at low tiers, the USSR planes have more guns.
.303 - UK. Somewhat more powerful than either of the 7.62 mm.
7.7x58 mm Arisaka - Japanese. Virtually identical to the .303 rounds (Wikipedia says they started by copying the British .303).
7.92x57 mm Mauser - Germany. Seems to be the most powerful non-12.7 mm machine gun round.
12.7x99 mm - USA. Historically known as ".50 cal." It is also very similar to the .30-06 Springfield hunting rifle cartridge. This seems to be the most powerful machine gun round in the game.
12.7x108 mm - USSR. Might be slightly more powerful than the 12.7 used by the USA, but seems about the same.
Only the USA and USSR used heavy machine guns. The UK, Japanese, and Germany began using autocannons earlier. The USA in particular loved heavy machine guns.
Judging from Wikipedia, it does not appear that the USSR ever used their popular 7.62x39 mm cartridge on aircraft.