Qfyilyi

Why do modern jets use 20mm guns? Is the armor of a modern jet that thick? Does, for example, .50 ammunition lack velocity to be effective?

I'm just curious on why is that 20mm is so widely used in air-to-air guns.

20mm is the smallest caliber that is practical to load with a bursting charge. Some fighter jets use 23mm, 27mm, or 30mm guns.

The reason 20mm has become one of the most common aircraft gun calibers is that it produces the lightest gun that meets the minimum requirements to be useful in air combat: enough rate of fire to hit a maneuvering jet, good enough ballistics for meaningful range, enough bursting charge capacity for meaningful damage.

The majority of modern combat jets do not carry armor, but they are built with considerable redundancy and internal subdivision. A bursting charge (explosive payload) is needed to do meaningful damage to a modern combat aircraft.

Smaller calibers can cause some damage that will require repairs, but not enough to rely on them to bring down a jet in combat. Larger calibers require either a heavier gun or a slower-firing one to keep the weight down. With missiles prioritized, most air forces want the lightest gun that's still effective.

The 20mm rounds used by combat aircraft have limited armor-piercing capability anyway (comparable to .50 BMG). Some armored exceptions include CAS aircraft such as the A10 and to a lesser extent the Su-25 and the Su-34. This armor is mostly designed to protect them against ground fire, not enemy fighters.

If you want to know why you put a caliber usually between 20 and 30mm on a fighter plane (if you do any at all) you have to consider a few things.

1. Modern fighter planes can achieve speeds of more than 600 m/s. Even in maneuvering combat - which today is rather non existant - they still move at 200 m/s.


2. Engagement distances are therefore (turning circles etc. included) at least 500m to 1000m or far far more.


3. Weight on a plane is very much limited due to structural components, fuel and mission payload (which includes bombs and guided missiles).

With that in mind think about what a gun can achieve:

Typical weight for a gun system of calibres 30mm and higher is in the range of several tons. The GAU-8/A Avenger (the A-10 gun) has a (dry) weight of 1800kg. That is without ammunition, which also adds considerable weight. Thus it limits the payload for other ordinance available. Anything larger is fairly impractical due to weight considerations.

You are fighting at high velocities and at long ranges. You need a gun that has a high range of fire and a high projectile velocity. Thus anything below a .50 caliber gun has troubles considering the ranges involved (projectiles slow down considerably if fired at longer ranges and need to be fired in a higher arc).

Even with a high rate of fire (say 6000 rounds per minute) you end up with a space of 10 meters in between every round at the respective muzzle velocities (~ 1100 m/s). Your target is also moving at a considerable speed, thus even if you fire a long burst at a passing plane, it is unlikely to hit with more than a handful of rounds - if you hit at all.

Thus you want to maximize the effect you have, which means you need a projectile with a bursting charge. Otherwise it might just pass through the enemy plane without dealing real damage. Control surfaces, tanks etc. are all not so much a problem with just a small hole in it. This means that you need a bursting charge - which needs a larger projectile, thus anything below 20mm is almost impractical, because the charge would be too small.

This leaves us with guns in (usually) the 20mm - 30mm range.

Almost all fighter plane and fighter bomber guns are in that range. A few examples:

• M61A1 Vulcan (US) - 20mm rotary cannon@6000 rounds per minute (the high rate of fire ensures more hits)


• Mauser BK-27 (DE) - 27mm revolver cannon@1700 rounds per minute (larger projectile, thus greater bursting charge)


• GSh-30-1 (RU) - 30mm revolver cannon@1500-1800 rounds per minute (again larger projectile but less rounds)


• GIAT30 (FR) - 30mm revolver cannon@2500 rounds per minute


• GAU-8/A (US) - 30mm rotary cannon@3900 rounds per minute (really high weight for gun system and ammunition, but isn't meant for air to air combat anyways)