50 cal (high rate of fire) vs 20mm cannon (hitting power)

[GregP]

If you Google "the great fighter gun debate," you can see tehre are several ways to consider guns.
1) For a machine gun, you have the kinetic energy times the firing rate minute. The Soviets divide this by the weight of the gun so they can factor in the gun installation.
2) For a machine gun, you could also evaluate the momentum. p = mv, so bullet mass times muzzle velocity.
3) For a cannon, you have the energy or momentum and then you have to figure in the elxplosive content. Some people take the percent explosive divided by ten, add it to 1 and then add that to the energy or momentum. There are several ways to account for the explosive content.

The point is, there are several ways to evaluate guns.

From what I've read, it takes about 2 - 3 times the number of machine guns hits to knock down a fighter versus 20 mm cannons, but that depends on where the hits are. Either a cannon or a MG round would likely pass through a wing tip without much damage. Either one would cause damage if it hit a wing attach point, but the cannon would do 2 - 4 times the damage.

Once you get to 30 mm cannons, there is no contest. One 30 mm cannon hit will lielly known donw a fighter unless it passes through an area that is just sheet metal with nothing in the middle to set off the round.

All we can say for sure is that there are no fighters around troday without cannons. The Soviet MiG-15 was a VERY hard-hitting airplane. It had two 23-mm cannon and one 37-mm cannon. If you got hit, you were likely going down or, at the least, you were out of the fight.

 

[pbehn]

But why define cannons by mass? I know that's naval tradition.

Why define by diameter, you can get a 0.5" handgun. A diameter and especially a metric measure gives a cloak of exactness, in fact, the diameter of a weapon is no more informative than the weight of the projectile.

 

[pbehn]

If you Google "the great fighter gun debate," you can see tehre are several ways to consider guns.
1) For a machine gun, you have the kinetic energy times the firing rate minute. The Soviets divide this by the weight of the gun so they can factor in the gun installation.
2) For a machine gun, you could also evaluate the momentum. p = mv, so bullet mass times muzzle velocity.
3) For a cannon, you have the energy or momentum and then you have to figure in the elxplosive content. Some people take the percent explosive divided by ten, add it to 1 and then add that to the energy or momentum. There are several ways to account for the explosive content.

The point is, there are several ways to evaluate guns.

From what I've read, it takes about 2 - 3 times the number of machine guns hits to knock down a fighter versus 20 mm cannons, but that depends on where the hits are. Either a cannon or a MG round would likely pass through a wing tip without much damage. Either one would cause damage if it hit a wing attach point, but the cannon would do 2 - 4 times the damage.

Once you get to 30 mm cannons, there is no contest. One 30 mm cannon hit will lielly known donw a fighter unless it passes through an area that is just sheet metal with nothing in the middle to set off the round.

All we can say for sure is that there are no fighters around troday without cannons. The Soviet MiG-15 was a VERY hard-hitting airplane. It had two 23-mm cannon and one 37-mm cannon. If you got hit, you were likely going down or, at the least, you were out of the fight.

From the picture of a Spitfire hit by 3 or 4 cannon rounds posted here a few times, the side of the fuselage opposite to where the hits occurred ceased to be a stressed skin structure, not only pierced by many fragments of shrapnel but also blown out like a balloon. The pilot was a very lucky man.

 

[swampyankee]

But why define cannons by mass? I know that's naval tradition.

Until shells became common, it was pretty much universal.

 

[pbehn]

Until shells became common, it was pretty much universal.

The weight (or mass) was pretty much all that mattered.

 

[PAT303]

From the picture of a Spitfire hit by 3 or 4 cannon rounds posted here a few times, the side of the fuselage opposite to where the hits occurred ceased to be a stressed skin structure, not only pierced by many fragments of shrapnel but also blown out like a balloon. The pilot was a very lucky man.

The plane was under full control the whole time and the pilot landed without incident, his only injury was shrapnel in his feet that went under the seat armor. Cannons are far more effective than MG's but only if the ammunition was reliable, the German 20mm ammunition used in the BoB was fitted with a graze fuse that was too sensitive which is the reason the three hits on the Spit detonated on the outer skin.

It's the buckle above the far right hit that ended it's career and caused it to become a workshop donor.

 

[msxyz]

The main deciding factor of cannon vs machine guns is in the 'terminal ballistics'. Since aircrafts are made mostly of aluminum it's safe to assume that any bullet travelling fast enough will pierce through skin, structure elements, tanks, etc and will go on wasting most of its kinetic energy in its arced trajectory down to earth. A hit to the engine block is likely to be more devastating and a 'lucky' hit to a half empty fuel tank full of gasoline vapors can also be devastating (especially if the bullet is a tracer or has a phosphor tip) but that's more a matter or luck. An African hunter caught off guard once killed a charging bull elephant with a .22LR rifle of his wife by hitting the poor animal straight through the eye, but that doesn't make the .22LR an elephant gun!

The reason behind having a volley of 4-8 machine guns firing a stream of light bullets at a high rate was that the bullets would act as a wire saw cutting loose large portions of the skin or even the structural trusses of the airplane. Also, without sophisticate predicting gunsights firing a lot of bullets increased the odds that some would hit a vital part of the plane (engine, pilot, tank): same as the good old days in the age of sails when ships fired round balls, sometimes bouncing on the sea surface, in hope of cutting masts, tearing holes near the waterline, etc...

As planes got sturdier, however, sometimes it would take several hundred bullets to down even a simple fighter plane and, most of the time, it was again the result of a 'lucky' hit to the poor pilot, a vital part of the engine, or the usual tank full of inflammable vapors. Any bullet smaller than 20mm cannot contain a HE package large enough to bring more devastation to the plan than pure kinetic energy was already capable of (Italians and Japanese had HE .50mm rounds that they weren't very effective, being designed mainly to set tanks on fire). With a 20mm you can however pack 8-10 grams of HE or even 17 if you design a thin walled shell like the Germans did. That's enough to make the shell burst into splinters that multiply the effect of the hit. Even a small charge (5-6 gr) is able to break the bullet into a handful of heavy fragments travelling in a cone volume starting from the point of impact.

In addition to that, a 20mm shell weighting 90-130 grams can also defeat the plane armor something that a .50mm machine gun cannot always reliably do. Keep in mind that most pilots fired their guns only when they were very close to the enemy plane; while a .50mm gun is a formidable killer even at 1+ Km distances and it doesn't suffer from trajectory drops as much as a 20mm shell fired from a oerlikon type gun (600-700m/s max), if your target is less than 300m away, you're not going to use the ballistic advantage of the .50mm boat taile bullet, so it's b better to have a slow moving heavy bullet with possibly a HE content.

 

[A.G. Williams]

.50mm

I think you mean ".5 inch" (which is 12.7 mm). There were a few aircraft with 50+ mm guns but they didn't see much use.

Those air forces with both 12.7 mm HMGs and 20 mm cannon generally found the cannon to be about three times as destructive.

 

[Glider]

The main deciding factor of cannon vs machine guns is in the 'terminal ballistics'. Since aircrafts are made mostly of aluminum it's safe to assume that any bullet travelling fast enough will pierce through skin, structure elements, tanks, etc and will go on wasting most of its kinetic energy in its arced trajectory down to earth. A hit to the engine block is likely to be more devastating and a 'lucky' hit to a half empty fuel tank full of gasoline vapors can also be devastating (especially if the bullet is a tracer or has a phosphor tip) but that's more a matter or luck. An African hunter caught off guard once killed a charging bull elephant with a .22LR rifle of his wife by hitting the poor animal straight through the eye, but that doesn't make the .22LR an elephant gun!

The reason behind having a volley of 4-8 machine guns firing a stream of light bullets at a high rate was that the bullets would act as a wire saw cutting loose large portions of the skin or even the structural trusses of the airplane. Also, without sophisticate predicting gunsights firing a lot of bullets increased the odds that some would hit a vital part of the plane (engine, pilot, tank): same as the good old days in the age of sails when ships fired round balls, sometimes bouncing on the sea surface, in hope of cutting masts, tearing holes near the waterline, etc...

As planes got sturdier, however, sometimes it would take several hundred bullets to down even a simple fighter plane and, most of the time, it was again the result of a 'lucky' hit to the poor pilot, a vital part of the engine, or the usual tank full of inflammable vapors. Any bullet smaller than 20mm cannot contain a HE package large enough to bring more devastation to the plan than pure kinetic energy was already capable of (Italians and Japanese had HE .50mm rounds that they weren't very effective, being designed mainly to set tanks on fire). With a 20mm you can however pack 8-10 grams of HE or even 17 if you design a thin walled shell like the Germans did. That's enough to make the shell burst into splinters that multiply the effect of the hit. Even a small charge (5-6 gr) is able to break the bullet into a handful of heavy fragments travelling in a cone volume starting from the point of impact.

In addition to that, a 20mm shell weighting 90-130 grams can also defeat the plane armor something that a .50mm machine gun cannot always reliably do. Keep in mind that most pilots fired their guns only when they were very close to the enemy plane; while a .50mm gun is a formidable killer even at 1+ Km distances and it doesn't suffer from trajectory drops as much as a 20mm shell fired from a oerlikon type gun (600-700m/s max), if your target is less than 300m away, you're not going to use the ballistic advantage of the .50mm boat taile bullet, so it's b better to have a slow moving heavy bullet with possibly a HE content.

There is a lot of sense in this but there are a couple of points where reflection may help
Since aircrafts are made mostly of aluminum it's safe to assume that any bullet travelling fast enough will pierce through skin, structure elements, tanks, etc and will go on wasting most of its kinetic energy in its arced trajectory down to earth. LMG's were not as good at this as you may first think. An LMG round can easily be deflected by an aircraft rib and its ability to penetrate fuel tanks with enough effect to damage the self sealing properties is low.
Keep in mind that most pilots fired their guns only when they were very close to the enemy plane Whilst I agree that they should have done, were told and ordered to. The vast majority of pilots fired at too far a distance.

 

[PAT303]

LMG's were not as good at this as you may first think. An LMG round can easily be deflected by an aircraft rib and its ability to penetrate fuel tanks with enough effect to damage the self sealing properties is low.

The RAF chose 8 LMG's because a 2 second burst put 300 projectiles into the enemy aircraft which gave the best chance of incapacitating the pilot, it's the reason why late war MkXIV's still carried the .303. Common myth say's the .303's weren't effective against German aircraft in the BoB because they were fitted with armor, what people fail to realise is the armor was fitted because of the number of pilot injuries and deaths suffered in the Battle of France. There are numerous accounts of planes making it back across the channel filled with dead and dying crew or piloted by a crew member that's not the pilot after being hosed by 8 gunned Spits and Hurri's.

 

[Glider]

The RAF chose 8 LMG's because a 2 second burst put 300 projectiles into the enemy aircraft which gave the best chance of incapacitating the pilot, it's the reason why late war MkXIV's still carried the .303. Common myth say's the .303's weren't effective against German aircraft in the BoB because they were fitted with armor, what people fail to realise is the armor was fitted because of the number of pilot injuries and deaths suffered in the Battle of France. There are numerous accounts of planes making it back across the channel filled with dead and dying crew or piloted by a crew member that's not the pilot after being hosed by 8 gunned Spits and Hurri's.

There is no doubt that before the German aircraft were fitted with armour the .303 was an excellent weapon. However once armour and self sealing fuel tanks were fitted then it wasn't nearly as effective.
Again you are partly right when you say There are numerous accounts of planes making it back across the channel filled with dead and dying crew or piloted by a crew member that's not the pilot after being hosed by 8 gunned Spits and Hurri's. I say partly as this did happen but if the 303 had been replaced by a heavier weapon, the German bomber wouldn't have made it back.

Lets take a simplistic view on this. There are a number of examples of German bombers making it back (shot to pieces I agree) after being hit by 200 303 bullets. . A 20mm had half the ROF fire of a 303 and a Hurricane could carry 4 x 20mm instead of 8 x 303. Thus 200 303 hits would equal 50 x 20mm. Show me anything that made it home after being hit by 50 20mm shells.

When you say it's the reason why late war MkXIV's still carried the .303. I am afraid that your wrong. Late Spit XIV carrier 2 x 0.5 not 4 x 303

 

[Shortround6]

With a 20mm you can however pack 8-10 grams of HE or even 17 if you design a thin walled shell like the Germans did. That's enough to make the shell burst into splinters that multiply the effect of the hit. Even a small charge (5-6 gr) is able to break the bullet into a handful of heavy fragments travelling in a cone volume starting from the point of impact.

The German thin wall shell depended on blast, It broke up into lots of little fragments and not very many big ones so that the distance from the explosion site tended to be rather important to damage done. The thicker wall shells with less explosive were rather a mixed bag. For best fragmentation (optimum size and number) the explosive used had to be matched to the steel alloy and heat treatment of the shell body. Mis matches would result in everything from clouds of dust to a very few big fragments not traveling very fast. Most shells were somewhere in between but it took quite a while to get rid of both the 'make the shell bodies as cheap as you can" and the "fill them with whatever you have available at the time" mentalities. Some countries had little choice at certain points in the war but there was an awful lot of less than theoretical optimum munitions.

In addition to that, a 20mm shell weighting 90-130 grams can also defeat the plane armor something that a .50mm machine gun cannot always reliably do.

This rather depends on the actual shell design. German thin wall shell didn't penetrate very well. Which is why they used a 115-117 gram shell in mixed belts with the Mine shell (mine shell also didn't adapt to tracer very well). Soviet and Japanese light 20mm AP shells are also going to have problems. A 20mm gun is trying to make a hole almost 2.5 times bigger in area than a 12.7mm projectile. That is a lot of material to remove. The 20mm needs to apply as much or more energy to the area of hole per unit as the 12.7mm will. Low power 20mm shells won't do it. Now throw in that most 20mm guns used different types of shells, the HE shells and the AP shells with much less explosive and the versions with tracer which had even less.

while a .50 CAL gun is a formidable killer even at 1+ Km distances and it doesn't suffer from trajectory drops as much as a 20mm shell fired from a oerlikon type gun (600-700m/s max), if your target is less than 300m away, you're not going to use the ballistic advantage of the .50 CAL boat taile bullet

It is not the drop that screws things up, it is the lead needed. If your .50 cal needs 1.5 seconds to travel 1000yrds your 300mph target will have moved 660ft. adjust as needed.
Bullet has not dropped anywhere near 600ft at 1000yrds, and if zeroed for even 250 yds or so in a P-38 it won't drop below the line of sight until 550-600yds. Yes it will be low at 1000yds but the main sources of error will be distance estimation, target speed/direction and amount of lead needed.

 

[XBe02Drvr]

(Italians and Japanese had HE .50mm rounds that they weren't very effective,

something that a .50mm machine gun cannot always reliably do

while a .50mm gun is a formidable killer

I guess so! A 50mm machine gun would be firing a projectile nearly two inches in diameter, and the recoil of that in automatic fire would shake just about any plane to pieces.
You don't suppose msxyz meant .50 CALIBER, do you?
OTOH, an actual .50mm round would be about the size of a needle point.

 

[Shortround6]

 

[XBe02Drvr]

View attachment 606448

Quite the proboscis! What is it?

 

[Shortround6]

One of three Me 262s fitted with a 50mm cannon

 

[XBe02Drvr]

One of three Me 262s fitted with a 50mm cannon

Awesome! Rate of fire, and how many rounds carried? And what's it's estimated service life? Artillery style full hydraulic recoil mechanism? Must be a heavy S.O.B.

 

[Shortround6]

MK 214A cannon - Wikipedia

 

[PAT303]

Again you are partly right when you say There are numerous accounts of planes making it back across the channel filled with dead and dying crew or piloted by a crew member that's not the pilot after being hosed by 8 gunned Spits and Hurri's. I say partly as this did happen but if the 303 had been replaced by a heavier weapon, the German bomber wouldn't have made it back.

No they wouldn't have, the shot gun effect of the eight .303's was an attempt to make up for a lack of shooting ability, that's why bombers flew back to France peppered with holes, the average pilot just aimed at big black blob rapidly filling up their windscreen and let rip, the use of cannons in that situation would just have pilots running out of ammo quicker and hitting nothing.

 

[PAT303]

When you say it's the reason why late war MkXIV's still carried the .303. I am afraid that your wrong. Late Spit XIV carrier 2 x 0.5 not 4 x 303

No you are wrong, 0.5 cals were only fitted after gyro gunsights became standard and reliable ammunition was developed, until that happened two
Hispano's with HEI and SAPI and four .303's with AP and Incendiary was the standard fitment.