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Why do modern fighter jets use 20mm guns? Is the armor of a modern jet that thick? Does, for example, .50 (12.7mm) ammunition lack velocity to be effective?

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

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    $\begingroup$ Welcome to SX! Please try to put a bit more effort into researching and formatting the questions. They are persistent, and should preferably be applicable to a broader audience. Keep the title simple and put your research into the post body. Unfortunately I couldn't understand your third question at all. $\endgroup$ – Therac Dec 12 '18 at 11:05
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    $\begingroup$ Remember that 20 mm canons were almost necessary even soon in the WWII. Only a minority of fighters kept .5 machine guns only and .3 machine guns only Spitfire I was too weak right from the beginning. To some extent that is due to armor that might not be used any longer, but just a bullet without an explosive charge simply does not do enough damage when hitting a plane. $\endgroup$ – Vladimir F Dec 12 '18 at 18:48
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    $\begingroup$ FWIW so as to not compare apples to oranges here, a .5 caliber barrel has an inner diameter of 12.7mm, and a 20mm cannon would translate into .78 caliber. Caliber being synonymous with inch in this context. $\endgroup$ – seizethecarp Dec 12 '18 at 19:57
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    $\begingroup$ Last time I flew on a jet, there were no cannons at all -- was that 737 defective, or did the airline just cheap out on the options? $\endgroup$ – Jules Dec 14 '18 at 5:48
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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. With missiles prioritized, most air forces want the lightest gun that's still effective.

Larger calibers have a cost in design. A large caliber rapid-firing gun will be much heavier, its propellant gases (muzzle exhaust) can cause flameouts in the engine, and firing will throw the aircraft's aim off center. A slower-firing one can be suitably light, but still has the same recoil force. Another issue with large calibers is collateral damage - 30mm+ rounds send fragments a couple hundred meters, far enough to damage friendly aircraft in air combat maneuvering.

The 20mm rounds used by combat aircraft have limited armor-piercing capability anyway (comparable to .50 BMG). Armored aircraft are rare, and some 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.

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    $\begingroup$ There is, in fact, armor on multiple modern jets; an easy example is the A-10. I assume you meant to narrow that statement to fighters, but I think the answer can be improved in that direction. $\endgroup$ – AEhere supports Monica Dec 12 '18 at 12:26
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    $\begingroup$ You could also say that the 20mm Vulcan hits a sweet spot for a lot of areas at once; rate of fire and hence hit probability, ammo load and hence firing duration available, and so on, all maximized by using the smallest calibre with decent lethality. Contrast it with the 37mm single barrel gun used on Soviet aircraft, where the hit probability was so low due to the low rate of fire, it negated most of its benefits. Rate of fire is everything in air to air. The German 30mm Mk108 was so effective because it has an unusually high rate of fire (600rpm IIRC) for such a large single barrel gun. $\endgroup$ – John K Dec 12 '18 at 12:27
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    $\begingroup$ @AEhere True, I was addressing just the most stereotypical scenario of air combat. Edited to expand. $\endgroup$ – Therac Dec 12 '18 at 12:44
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    $\begingroup$ Therac: figured as much +1'd @JohnK wasn't the 37mm used in conjunction with 20mm, as was usual in WW2 and early jets? Nowadays the Su-27 uses 30mm ammo with at over 1500rpm, still slow compared to the 6000rpm of the Vulcan. $\endgroup$ – AEhere supports Monica Dec 12 '18 at 12:52
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    $\begingroup$ @Anoplexian The force and the vibration of the recoil, as well as the off-center effect of tilting the aircraft throwing the gun's aim off, and propellant gas ingestion, are simply much greater design concerns than direct thrust loss from recoil. If, say, the A-10 had 10 kN or 100 kN engines, it wouldn't directly affect its usable gun size. Being a larger tougher or smaller lighter-built aircraft would. Anyway, I've elaborated on this in the answer. $\endgroup$ – Therac Dec 14 '18 at 8:18
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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 ordnance 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 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)
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    $\begingroup$ Technical correction: The GSh-30-1 is a rapid-fire single chamber recoil operated autocannon, not a revolver. The GSh design is exceptional in having the rate of fire of cannons twice its weight, but pays for it in being heat-limited. Also, one important factor with Gatlings is that the stated rate of fire is maximum, not average - much lower during the spin-up time. The GAU-8 stands alone in being a real gun for repeated prolonged firing, rather than a fighter weapon meant to only fire a few bursts. $\endgroup$ – Therac Dec 12 '18 at 20:17
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    $\begingroup$ You were probably thinking of GSh-6-30 (which is always worth mentioning; like GAU-8, its a peak gun in many respects). That's a 6-barrel revolver (with 5000+ rpm!) But then, see @Therac's comment. Its total live is less than MTBF (jams) of the Vulcan. $\endgroup$ – Zeus Dec 13 '18 at 2:15
  • $\begingroup$ @Zeus You're both somewhat right. I mixed up the guns somehow, since GSh sadly has built a lot more 30mm cannons. $\endgroup$ – Adwaenyth Dec 13 '18 at 6:28
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One should remember also "ballistic drop" because that greatly complicates shooting a moving target at distance. And fighter jets are already moving fast. Narrowing the "3D-ness" of where the round should end up makes it easier to be far more accurate, one imagines, and gives more time for other considerations a pilot may have...

Just going on the assumption a round is the same length, whether 20mm or 30mm, which does not seem to be so, resulting in ONLY the increased diameter of perhaps similarly weighted materials to be important, which does not seem to be so (but both "not so's" seem to be conservative), a 30mm round would weigh 2.25 times what a 20mm round weighs. That would mean, if the energies at firing were able to be equal, unless one chose to make them different, that the "velocity squared" term would be 2.25 times greater, or in other words, that velocity would be 1.5 times greater. Then, ignoring that a smaller round ought to confront less wind resistance and therefore maintain its velocity longer (in time) than a bigger round, the 20mm round will be faster at all points along its flight path so it will reach whatever target in 2/3 the time the bigger one will.

Remembering Galileo, both dropping due to gravity at the same rate, the shorter flight time will result in less drop, accentuated by the fact that drop accelerates as time increases. In fact, a small (small) amount of research indicates it might be about 4/7, call it 55% of the larger round's drop.

Given the conservatism and ignoring how independent choices might make some of it moot anyway, I'm comfortable moving on with the idea of the drop being 1/2. So your round might fall, say, 200 feet below the other aircraft instead of 400 feet. Both can be planned for and a gun can be mounted to make it look like a laser shot for a pilot (at a single, chosen range), but it seems targeting a target might be a LOT easier to be accurate about when allowing for the much smaller drop. Also, the greater the drop, the higher above the target one must aim and therefore the longer the parabolic path the round must take increasing the loss of velocity vs. the smaller round, and on and on...

Easier to at any given accuracy means quicker, less time spent on its mechanics, and so, the extra second here and there. Seems like those seconds could matter a lot on their own.

Penetrating power would strongly depend upon energy content at impact for any given material/geometry. The round that takes about 2/3 the time to "get there" seems, by the same small (small) amount of research, to have 4/3 the energy at impact making it likelier that it would penetrate so that the bursting would be internal where it would have far greater effect: more ripping up flesh, electronics, flesh, fuel, flesh, hydraulics, flesh, radar profile, flesh... and less just messing up the paint.

Using the quick example in another Answer, I get a 36 foot separation between rounds. It seems to me that tracking the enemy so as to impact it with more than one round would be easier having a radically smaller drop to factor in. But I will say that it seems unlikely the designers planned for more than one round to hit as surely 2-3 guns firing at the same instant, a foot or two apart perhaps would be wonderful if they thought mor ethan one round was incredibly worthwhile and that just is not how the fighters are built so... Still, missing a few and tracking in on the enemy would seem easier as well with the smaller drop envelope.

Finally, the shorter time from firing to impact would have some smaller effects, like leaving less time for the enemy to maneuver out of the shot ("dodge")(realistically, more time for whatever instant thing he did to work, if it ever could... fewer of those extra seconds I mention above), like being able to take a meaningful shot on less time in frame (so taking a shot vs. "D*mn, too late to even take a shot"), like being able to fire in aid of a team member rather than worrying it might hit him, and other things that are even less quantifiable.

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  • $\begingroup$ All of your calculations seem to be predicated on the idea that a 20mm round and a 30mm round would have the same kinetic energy at launch. But is that a reasonable assumption? A larger round would surely be equipped with a larger charge, leading to more KE at launch. One could presumably, in principle, arrange for the muzzle velocity of the two rounds to be equal, at which point all your calculations disappear, since the only difference would be the larger round having more air resistance, which you mention but don't include in your calculations. $\endgroup$ – David Richerby Dec 13 '18 at 10:36
  • $\begingroup$ @DavidRicherby Force exerted on a projectile through a barrel increased with the square of the diameter, mass with the cube. Thus to achieve the same muzzle velocity, pressure has to be higher on the larger projectile or the barrel longer. Higher pressure means thicker barrel with more mass. Longer barrel also means more mass. Both things are detriminal in terms of mountability on a platform already strapped for total mass. But yes, increasing propellant charge and barrel length can lead to higher muzzle velocity. Compare a 120mm tank shell @ 1700m/s+ on newer models. $\endgroup$ – Adwaenyth Dec 13 '18 at 11:44
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    $\begingroup$ @Adwaenyth Except this answer considers none of that. Also, it makes the simplifying assumption that the 30mm round is the same length as the 20mm one, which means that mass is also only increasing with the square of diameter so, under that assumption, it seems that the 30mm round would have exactly the same muzzle velocity as the 20mm round. So the calculations here don't seem realistic at all. $\endgroup$ – David Richerby Dec 13 '18 at 12:19
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    $\begingroup$ While this answer does state its hypotheses before calculating, I find those oversimplified and, to be honest, pointless... the muzzle velocity and shell geometry of the most common 20mm and 30mm A/C gun rounds are either publicly available or easy to approximate, with no need to resort to spherical cows on infinite planes. $\endgroup$ – AEhere supports Monica Dec 13 '18 at 12:39

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