World War II fighter armament: what was too light, what was overkill, what was the Goldilocks zone?

[z42]

A (very rough, but bear with me here..) rule of thumb I've seen is that for each step down in (the widely used) calibre, about 4-5x as many hits were needed to bring down a plane. For a single-engined plane, a single 30mm hit was usually enough to bring it down. Step down to 20mm, and you need about 4-5 hits. Step further down to HMG class guns, and you need a further 4-5x, that is, around 20 hits. Step down all the way to rifle caliber, and you need around a hundred hits. Bigger aircraft were obviously sturdier, I think the average was something like 4-5 30mm hits to bring down a 4 engine heavy bomber. Now these rules of thumb are AFAIU roughly based on averages of shot down planes, obviously you can find outliers in all directions.

With that in mind, for shooting down fighters anything above 30mm is probably a waste. Even 30mm is a bit on the heavy side, as a 30mm gun with decent muzzle velocity and rate of fire would be heavy enough to be a significant problem for installing on a WWII fighter. Stepping down to 20mm seems to have, historically, been a pretty good compromise, as can be seen by the late war fighters with 4x20mm armament. One problem with 20mm is that it's slightly marginal for shooting HE shells in that the ballistic coefficient tends to be slightly poor (particularly for mine shells), and an annoyingly large fraction of the internal volume will be taken up by the fuze rather than HE filler.

So perhaps an optimal choice would be something in the 25mm range? Such as the Madsen mentioned by marathag . Though muzzle velocity and RoF is somewhat worse than one would have liked.

(That is not to say the US choice to standardize on the Browning HMG was a poor one. There are certainly big logistical advantages to that, though the gun itself was maybe not that impressive in terms of punch per installed weight. But then again US fighters tended to be on the bigger side in general, so they had better ability to carry a heavy battery to compensate. And of course, the US had little need to shoot down heavy bombers, where a big autocannon would really shine.)

 

[BarnOwlLover]

I'd have to say that, at least from 1942-45, 4 20mm cannons or maybe 2 .50s and 2 20mm would've been optimal for a fighter the size of say a Spitfire or Mustang. At least 4 20mm would be optimal for a plane the size of a P-47 or Tempest.

The other issue is ammo capacity. Spitfires that carried 20mm cannon (until the 20 series) carried 120 rounds per gun typically. When they carried .50s they carried 250 rpg. The cannon armed Mustang Is carried 125 rpg. The XP-51F/G was intended to carry 250 rpg for 4 .50s or 125 rpg for 4 20mm cannons. In the P-51H its 6 .50s carried 390/260/260 rpg (the P-51D carried 400/270/270).

One maybe strange aircraft was the Republic XP-72. It went from the 8 .50s of the P-47 with 425 rpg to 6 .50s and 270 rpg. Even though engine power went up significantly vs the P-47D of the period due to replacing the P&W R-2800 with the R-4360.

Another was the XP-67, which went from 6 20mm cannons with 165 rpg and 6 .50s with 500 rpg, to ultimately either 6 37mm cannons with 45 rpg, or 2 37mm and 4 20 mm cannons. But the XP-67 was a dedicated bomber destroyer, while the XP-72 was intended to be an interceptor, though for bomber destroying even it was intended to have provision for 4 37mm cannons replacing the .50s.

Of course, with the gyro gun sight, ammo capacity mattered less because of vastly improved accuracy. It also did a better job of compensating for ballistics of the armament.

But this also shows the differences between "light" GP fighters or interceptors, vs "heavy" fighters and interceptors even within the single seat/single engine class.

 

[Shortround6]

It is very, very rough.

Single engine fighters can go from 6,000lbs (or under) to 12-13,000lbs (or over) some single engine bombers can go a bit higher.
twin engine bombers can go from 10,000lbs (Blenheim I) or under to close to 40,000lbs (Manchester and Warwick and ?)

4 engine aircraft.............generally start at 40,000lbs and go up, odd ball French prototypes/low production stuff aside.

Divorcing individual hit damage to rate of fire is hard.

Guns from the bottom up.

RCMG (Rifle Caliber Machine Guns) are pretty close in power (bullet weight and veleocity) however the rate of fire can vary by 3 times. Lewis gun to ShKAS.

Next step up is the light 12.7-13mm guns. These are about 3 times as powerful per shot as the RCMGs. Their rate of fire makes them about 1.6 to 2.1 times as powerful as the middle RCMGs (1200rpm)

Next is the heavy 12.7-13.2mm guns. These are 4.5-6 times as powerful per shot as the RCMGs. Combined with the rate of fire these are about 3 times (US .50) to 4 + times (Soviet 12.7mm) as powerful as the RCMGs.

20mms are all over the place The least powerful 20mm is about 8 times more powerful than the RCMG while the most powerful is 20-22 times as powerful per shot. With rate of fire the difference is about 4 1/2 times to 11 times as powerful per gun.

The German short 30mm is about 60 times as powerful per hit and about 24 times as power figuring in the rate of fire.

WEIGHT OF THE GUNS & AMMO NOT CALCUALTED IN !!!!

I believe, but could very well be wrong, that the early Gyro gun sights only corrected for lead. This requires the guns to all have about the same velocity and the Gyro only measured the turn rate of the aircraft. The gun sight had no way of measuring the range aside from the pilot adjusting a ring (guessing) and depended on the pilot keeping the target inside the sight while the sight gyro figured out the rate of turn. This could take a 1/2 second or more and depended on the pilot being somewhat honest. If the target was bouncing all over the sight in a high speed jitter the result wasn't going to be worth much.

Some of the American proposed armaments using the 37mm gun may have been wishful thinking. The Belt feed version didn't show up until the A-9 version of the P-63 much of the way through 1944. Gain in rate of fire was about 20rpm.

 

[z42]

Oh, and one thing I forgot above, I think there was an advantage to having a uniform armament on a plane. Identical ballistics, and one less thing for pilots to worry about (which guns should I use in this situation, or just blast away with everything I have hoping that at least something hits?), and of course the logistical advantages as well. That was an advantage of the all-HMG armament the US fighters preferred over the mixed MG + cannon armaments many others used, to some extent compensating for the lower punch per weight of the Browning HMG.

 

[Shortround6]

Oh, and one thing I forgot above, I think there was an advantage to having a uniform armament on a plane. Identical ballistics, and one less thing for pilots to worry about (which guns should I use in this situation, or just blast away with everything I have hoping that at least something hits?), and of course the logistical advantages as well. That was an advantage of the all-HMG armament the US fighters preferred over the mixed MG + cannon armaments many others used, to some extent compensating for the lower punch per weight of the Browning HMG.

For the US and British the difference in ballistics (drop and time of flight) was so close at up to 600yds as to make little practical difference. The differences increase as the range opens up and can be substantial at ranges between 1000-2000yds but no pilots had any business trying to shoot at 1000yds, didn't keep some of them from trying

 

[z42]

For the US and British the difference in ballistics (drop and time of flight) was so close at up to 600yds as to make little practical difference.

The 20mm Hispano and the .50 BMG were pretty well matched ballistically, yes. Some other cannon + MG combinations, not so much.

(IIRC the ballistic matching was one of the main arguments for changing the mid-late war Spits from 2x20mm + 4x.303 to 2x20mm + 2x.50.)

 

[z42]

The USN found it needed to upgrade its AA armament for Kamikazes which had to be blown out of the sky, not just shot down and continue on a ballistic path until crashing (into a ship).

For AA the factors affecting the "optimal" choice of gun size are a bit different, so the result ends up a bit different as well. Due to the velocity difference between the plane and the AA gun, you want more range so you have more time to pump shells into the air before the target flies out of range. Also the gun platform is more stable than a plane, and weight is less of a concern, all driving the optimal point towards bigger guns.

All this to say the 40mm Bofors was exactly the right tool for the job, and is justifiably called the best medium AA of the war. Light AA, like the ubiquitous 20mm Oerlikons, were somewhat marginal already at that point, but you can fit only so many Bofors guns on a ship, and sprinkling Oerlikons in every corner where you couldn't fit a Bofors was better than nothing.

 

[Shortround6]

For AA the factors affecting the "optimal" choice of gun size are a bit different, so the result ends up a bit different as well. Due to the velocity difference between the plane and the AA gun, you want more range so you have more time to pump shells into the air before the target flies out of range. Also the gun platform is more stable than a plane, and weight is less of a concern, all driving the optimal point towards bigger guns.

All this to say the 40mm Bofors was exactly the right tool for the job, and is justifiably called the best medium AA of the war. Light AA, like the ubiquitous 20mm Oerlikons, were somewhat marginal already at that point, but you can fit only so many Bofors guns on a ship, and sprinkling Oerlikons in every corner where you couldn't fit a Bofors was better than nothing.

Just missing the war the 3in AA gun with auto loader and proximity fuse was the best solution. It was being worked on during the war but just missed.

" with the first prototype being ready for test firing on 1 September 1945. The end of the war slowed development and resources were diverted to the more potent 3"/70 (7.62 cm) design. As a result, it was not until 1948 that this weapon was delivered to the fleet in quantity."

But you can't even fit a 40mm Bofors into an airplane. However you can get a fast firing quad 20mm into an airplane and get it fairly close

 

[marathag]

The Browning 0.5 inch machine gun was just about perfect for what the USAAF encountered in WW2, it would not be its fighters armement if the USAAF was tasked with shooting down B-17, B-24 and B-29.

So much effort wasted on the .60 and .90 caliber efforts, the errors in the 20mm development too.
Go across the Pacific, had the Japanese that just kept making their .30 Brownings larger and larger as the War went on.
The USAAC and the USN had brief flirtation with the Madsen cannons(and 37mm) for expected use against enemy bombers before the War, then turning away and only worshiping at the altar of the 50 BMG, with a few underfunded heretics working on the other ideas.

And still took them years to get the RoF reliable for the .50 M3

Embarrassing.

 

[Shortround6]

And still took them years to get the RoF reliable for the .50 M3

Embarrassing.

True, but maybe they were reacting to the troubles they had with the M2 .50 cal and the 20mm.

Memory may be off but I seem to remember reading about 5 jams per 1000 rounds and about 1-2 broken parts as the max?
Granted test ranges are different than real life but maybe they could have backed off a little on the reliability ?

Granted a few never made it to even 500 rounds without blowing up which is another thing all together.

 

[BarnOwlLover]

Ironically, it seems that if the US built the HS-404 to the same tolerances that the British did (ie, looser), that could've resolved most of the issues.

There's also the odd issue that the US Army (and by extension the USAAF at the time) considered anything over .60 cal to be a cannon, and hence artillery, and any automatic weapon below that to be a MG.

Maybe an argument that the USAAF should've been more separate from the Army (or maybe just a standalone USAF like post-1947) before the war?

 

[z42]

Just missing the war the 3in AA gun with auto loader and proximity fuse was the best solution. It was being worked on during the war but just missed.

" with the first prototype being ready for test firing on 1 September 1945. The end of the war slowed development and resources were diverted to the more potent 3"/70 (7.62 cm) design. As a result, it was not until 1948 that this weapon was delivered to the fleet in quantity."

But you can't even fit a 40mm Bofors into an airplane. However you can get a fast firing quad 20mm into an airplane and get it fairly close

Yes, I didn't mention the 3" as it missed the war.

AFAIU the 3" size was chosen because it was the smallest shell they could fit a VT fuse into given the technology at the time. Medium AA at the time, like the 40mm Bofors, was contact fused, so a fast firing and training gun firing VT shells was a big improvement. Of course nowadays we can fit proximity and computer programmed time fuses into medium AA sized shells.

 

[pbehn]

With all the variables of availability, reliability, destructive power, weight, volume, drag and so on coupled with the mission requirements, I cant think think of any plane that was carrying armament that was substantially "wrong", it may have been a long time ago, but generally the people who made the decisions weren't fools, some may even have known az mutch az wot I doez.

 

[marathag]

Ironically, it seems that if the US built the HS-404 to the same tolerances that the British did (ie, looser), that could've resolved most of the issues.

There's also the odd issue that the US Army (and by extension the USAAF at the time) considered anything over .60 cal to be a cannon, and hence artillery, and any automatic weapon below that to be a MG.

Maybe an argument that the USAAF should've been more separate from the Army (or maybe just a standalone USAF like post-1947) before the war?

Separate or not, would still have to deal with the clownshow that Army Ordnance was with arms development of both MGs and Artillery.

 

[Shortround6]

I cant think think of any plane that was carrying armament that was substantially "wrong",

Well, I can
although many of these planes were constrained by either low powered engines which constrained load availalabitly due to performance or lack of manufacturing capability to build enough guns.

Something else that is not explored quite as much is the fact that it is projectiles that are the actual weapons. The guns are actually just the launchers.

The actual effectiveness of smaller sized guns is also often over looked or criticized unduly. The .303 is often quite heavily criticized and called a pop gun.
A single .303 hit is quite unlikely to cause the loss of an aircraft, golden BBs aside but in British fighters the idea was not to depend on one hit but on scores of hits.
A modern mini gun fires at 6000rpm or 100rps. Just 5 .303 Brownings will equal that and the Hurricanes and Spitfires had 3 more guns, 60% more fire power.
A Hurricane IIB had the firepower of TWO Mini guns and two extra machine guns.

Problems were lack of gunnery training, less than optimum firing patterns at times (trying to make up for the poor training) and lack of good projectiles.

As far as the guns themselves go eight .303s are pretty much the same fire power as 4.7 12.7mm Italian guns or about 3.7 German 13mm or Japanese 12.7mm guns.
Eight .303s are also pretty close to the firepower of a pair of 20mm Oerlikon FF cannon, so the 109E-4 and Zero only had about a 20-30% firepower advantage, which disappeared rather quickly. For some inexplicable reason Japanese 7.7mm/.303 guns/ammo is deadly while British 7.7mm/.303 guns/ammo only scuffs the plane and can be buffed right out
The British increased the proportions of INC and AP ammo after the BoB (supply problems) about the same time that the 20mm began to show up in quantity.
In 1942 a Spitfire was carrying more incendiary ammo for it's 4 guns as the 1940 Spitfire was carrying for 8 guns, The 1942 Spitfire was carrying almost an equal amount of AP.

Against the US .50 M2 the RCMG Browning was about 1/3 as powerful, but weighed about 1/3 less. The .50 did do more damage with a single hit. However in M2 in 1940 was slower firing and used less powerful ammunition. In 1941 the British were getting the less powerful ammunition and had a lot, an awful lot, of trouble getting the guns to run. IN 1942 the US was getting the better ammo but the USN was having trouble getting the guns to run. The US overloaded the planes with guns and ammo and hurt performance.
A P-40E with 6 guns and 235rpg was carrying 807lbs worth of guns and ammo. A Spitfire II/Va with eight .303s and 350rpg was carrying 440lbs worth of guns and ammo.
There was room for ammo in the P-40 ammo bins.

 

[marathag]

A Hurricane IIB had the firepower of TWO Mini guns and two extra machine guns.

It had a large number of .303 from individual low rate of fire, and then the issue of dispersion, and what range the guns were harmonized at, than the 'lance of steel' that a gatling gives.

Problems were lack of gunnery training, less than optimum firing patterns at times (trying to make up for the poor training) and lack of good projectiles.

Now Cowl and thru hub cannon mounts did not suffer from that choice of harmonization range and tight or wide grouping choices.
Does come up, if the massed .303 was so awesome, why was the RAF moving away from 12 gun 303 setups to a cannons?

 

[pbehn]

Well, I can
although many of these planes were constrained by either low powered engines which constrained load availalabitly due to performance or lack of manufacturing capability to build enough guns.

Something else that is not explored quite as much is the fact that it is projectiles that are the actual weapons. The guns are actually just the launchers.

The actual effectiveness of smaller sized guns is also often over looked or criticized unduly. The .303 is often quite heavily criticized and called a pop gun.
A single .303 hit is quite unlikely to cause the loss of an aircraft, golden BBs aside but in British fighters the idea was not to depend on one hit but on scores of hits.
A modern mini gun fires at 6000rpm or 100rps. Just 5 .303 Brownings will equal that and the Hurricanes and Spitfires had 3 more guns, 60% more fire power.
A Hurricane IIB had the firepower of TWO Mini guns and two extra machine guns.

Problems were lack of gunnery training, less than optimum firing patterns at times (trying to make up for the poor training) and lack of good projectiles.

As far as the guns themselves go eight .303s are pretty much the same fire power as 4.7 12.7mm Italian guns or about 3.7 German 13mm or Japanese 12.7mm guns.
Eight .303s are also pretty close to the firepower of a pair of 20mm Oerlikon RR cannon, so the 109E-4 and Zero only had about a 20-30% firepower advantage, which disappeared rather quickly. For some inexplicable reason Japanese 7.7mm/.303 guns/ammo is deadly while British 7.7mm/.303 guns/ammo only scuffs the plane and can be buffed right out
The British increased the proportions of INC and AP ammo after the BoB (supply problems) about the same time that the 20mm began to show up in quantity.
In 1942 a Spitfire was carrying more incendiary ammo for it's 4 guns as the 1940 Spitfire was carrying for 8 guns, The 1942 Spitfire was carrying almost an equal amount of AP.

Against the US .50 M2 the RCMG Browning was about 1/3 as powerful, but weighed about 1/3 less. The .50 did do more damage with a single hit. However in M2 in 1940 was slower firing and used less powerful ammunition. In 1941 the British were getting the less powerful ammunition and had a lot, an awful lot, of trouble getting the guns to run. IN 1942 the US was getting the better ammo but the USN was having trouble getting the guns to run. The US overloaded the planes with guns and ammo and hurt performance.
A P-40E with 6 guns and 235rpg was carrying 807lbs worth of guns and ammo. A Spitfire II/Va with eight .303s and 350rpg was carrying 440lbs worth of guns and ammo.
There was room for ammo in the P-40 ammo bins.

But were they conspicuously wrong? A spread pattern that compensates for lack of training is logical when your pilots arenet trained and training, if you can do it takes a long time. The Spitfire MkII was introduced during the BoB take off run and climb were important, so weight was too. Not or rarely mentioned in this are gun sights, which changed gun effectiveness as much or more than a change of gun did.

 

[BarnOwlLover]

Not to mention that if you wanted more firepower and range (might not mean much with wing mounted guns set to converge at a certain range) than what rifle caliber MGs There comes a point where 4 20mm cannons is just as weight effective as 6 50s. I did a calculation of where say 4 20mm Hispano cannons with 200 rpg weigh less than 6 .50 Brownings with 400 rpg. And that was for the 100+ lb Mk II Hispano cannons, the gap increases for the lighter Mk V.

Before 1942, most fighters had 1000-1300 hp engines. The fuel and supercharging (and other engine tech) wasn't there yet until 1942 really, and even then it took almost until the end of the war to reach its zenith. After that point, I'd say that for a plane with the P-51, 6 .50s with like 270 round for each gun was good, 4 20mm cannons with 140-150 rpg would probably have been better. Again, how many guns and how much ammo is overkill, or not enough?

And of course, there were reasons why there was a move to wing mounted guns. They didn't have to deal with rate of fire getting reduced due to prop synchronization. This became worse when especially Allied fighters went from 3 bladed props to 4 or sometimes (Griffon Spitfires) 5 bladed props. Only fighter that saw wide service with cowl mounted guns that had a 4 or more bladed prop was the Ki-84.

Also, the wings could hold more guns, and more ammo. The MiG I-225 prototype had 4 20mm ShVAK or B-20 cannons that were nose mounted, but only carried 100 rpg for them due to packaging reasons.

So basically until jets enter the picture, it's sort of pick your poison.

 

[Shortround6]

It had a large number of .303 from individual low rate of fire, and then the issue of dispersion, and what range the guns were harmonized at, than the 'lance of steel' that a gatling gives.

Well, the "lance of steel" seems to be 6.5 mils at 80% (GE literature for the SUU-11B-8 A Minigun pod) and since 6.5mils is about 22 inches at 100 yds (300ft) or 88inches at 400yds (1200ft) I would say that the "lance of steel" may have a bit of flaw.

Now Cowl and thru hub cannon mounts did not suffer from that choice of harmonization range and tight or wide grouping choices.

What they sometimes have trouble with is the different trajectories and times of flight for the different caliber guns. Cowl guns are generally limited to two guns (Russians used 3 and some times 4) and the hub guns were almost always a different caliber. Sometimes they played well together and sometimes they didn't. The difference in types of guns is not very important if you are at close range (short time of flight).

Does come up, if the massed .303 was so awesome, why was the RAF moving away from 12 gun 303 setups to a cannons?

Probably for several reasons.
The manufacture of 12 .303 guns was not cheap. The parts are not large but they are high quality, well fitted guns that require a lot of machining.
They also require a lot of maintenance. Clean, lube and load 4 big guns or 12 small ones?
And by the time you get to the 20mm Hispano each gun is about 10 times more effective than the .303 Browning.
Which is substantial, but it is not the 16 to 25 times more effective that z42 estimated in post #21.
Each 20mm Strike is about 20 times more effective than a .303 strike but the .303 Browning fires twice as fast.

Now on a per pound (kg basis) the 20mm Hispano gun was about twice as effective as the .303 Browning.
240 rounds of 20mm was equal in weight to 2260 rounds of .303 but the 20mm round hits harder as mentioned above.

Moving to the 20 Hispano was certainly the correct choice once they had more powerful engines. However the sort of revisionist narrative that the .303 was useless needs a lot of looking at.


I am using the .303 Browning as a best case. A .303 Lewis gun (or Japanese Navy type 92 MG) has got 1/2 the fire rate and 1/2 the effectiveness, assuming it can be aimed equally and uses equivalent ammunition.

 

[wuzak]

Another was the XP-67, which went from 6 20mm cannons with 165 rpg and 6 .50s with 500 rpg, to ultimately either 6 37mm cannons with 45 rpg, or 2 37mm and 4 20 mm cannons. But the XP-67 was a dedicated bomber destroyer, while the XP-72 was intended to be an interceptor, though for bomber destroying even it was intended to have provision for 4 37mm cannons replacing the .50s.

6 x 37mm would have been silly, and probably overkill.

Even though it wasn't a great aerial gun.

6 x 20mm would have been more than enough to deal with most threats.

Maybe a gun in the 25-30mm class would have been a good size for the expected role of the XP-67.


Also, the proposed Supermarine Type 327 had 6 x 20mm cannon. Probably overkill for the time, and the gun arrangement was one of the areas of criticisms the Air Ministry had for the Type 327 when evaluating the proposal.

4 x 20mm would have been better, and would give better packaging. The Hispanos were drum fed at the time of the proposal, so ammunition was limited.

The Type 327 was based on the Type 324, which had 12 x 0.303" mgs in the outer wings and was a competitor to the Hawker Tornado and Typhoon proposals,