Guns & Cannon: Rules of Thumb

[Glider]

Yes, it's true that the F4F-4 got two more guns due to British requests. Combined with the folding wing, performance degraded due to greater weight for no HP increase. .

This is a claim that is often made but as far as I can tell never verified. It also lacked logic.
The FAA had experience of using the Wildcat in combat from Late 1940 with the first kill happening on Christmas day 1940 and it was recognised from the start that 4 x 0.5 were far superior to the standard 8 x LMG used in the RAF and FAA. The 8 x LMG were more than good enough to deal with the Italian forces so there was no need to up the firepower from 4 to 6 HMG. If the UK forces had asked for an increase in firepower then its more likely that they would have asked for 2 x 20mm as the UK already decided to go for the 20mm as the standard fighter weapons. That would probably have been feasible as the paired 0.5 were close to each other (and an interesting thought).
I would expect that the RN would have asked for the folding wings after all they were supposed to fly from carriers and no doubt the USN would have wanted it for their carriers. Its a bit surprising that they didn't have folding wings from the start.

The only nation outside the USN was the French as they asked for their Wildcats to be armed with 6 x 7.5.

 

[parsifal]

Until the wing folding could be introduced, the martlets were considered generally unsuitable for fleet protection. Deck space on RN carriers was severely limited, and the non wing folding martlets could not be stowed in the hangars below decks. This had two effects, firstly that only a maximum of six Martlets could be carried by the Illustrious class and secondly aircraft turnaround rates suffered because of the overcrowding on the decks. Because they could not be stowed except on the upper decks they were considered an extreme fire hazard.

On board the CVEs where most of the CAG was a permanent deck park anyway, at least for the early CVE designs, this penalty was accepted

I'm reasonably sure the F4F4 arose from RN requests, but will be interested to see if that belief stands up. My suspicion is that 20mm were not fitted because of weight and strength issues. The RAF was having a hard time fitting the 20mm weapon into Spits and hurricanes from memory. Perhaps there wasn't time to put into effect a similar modification for the Martlet.

 

[Shortround6]

Timing can be critical. The early Spit Vs having drum fed guns. The Hurricane did not get 20mm guns until summer of 1941.
Ordering F4Fs in 1940 with 20mm guns before the belt feed for the Hispano is sorted out would be pretty much a non-starter. 6 seconds of firing time for a fleet defense fighter is not enough. The six gun F4F had 24 seconds if the guns were the old 600rpm guns and 18 seconds for the fast firing guns.

A few months (or even weeks) can mean the difference between ordering one version or another and sometimes they didn't have the luxury of waiting.

 

[yulzari]

At this site (.50 inch Browning - British Military Small Arms Ammo) one can see the British take on 0.5 Browning ammunition.

 

[David Fred]

I'm curious if there's a generalized metric when it comes to guns and cannon (allies at least)? I've generally seen figures similar to this

• 8 x 0.303, or...
• 6 x 0.50, or...
• 4 x 20mm, or...
• 2-3 x 30mm, or...
• 2 x 37mm/40mm...

It's clearly not perfect but the idea sort of came out of observations, as well as discussion with others.

• Hawker Hurricane (1935/11/6)
  • Designed with 8 x 0.303 to allow it to deliver enough bullets onto a bomber in 2-seconds
  • Later variants were developed with 4 x 20mm cannon
  • There was one variant built that had 2 x 40mm cannon
  • Exceptions: There was a variant built that had 12 x 0.303 and no cannon equivalent with 6 x 20mm
• Supermarine Spitfire (1936/3/5)
  • Designed with 8 x 0.303 for the same reason as the Hurricane
  • Proposals were made at some point for 6 x 0.50, and even something like 2 x 30-40mm cannon
  • Variants were built that carried 8 x 0.303, 2 x 20mm + 4 x 0.303; 4 x 20mm
    • Exceptions: The predecessors to the B-wing had only 2 x 20mm instead of 4 x 20mm, and the final set-ups had provision for 2 x 20mm + 2 x 0.50
• Bell P-39 (1938/4)
  • The P-39D had 4 x 0.303 in the outer wings, later replaced on some models with 2 x 0.50
• Vought F4U
  • 6 x 0.50 operational variants except one or two that had 4 x 20mm
• Bell YP-59 (1942/10/1)
  • One set-up had 1 x 37mm + 3 x 0.50; another had 2 x 37mm
• Grumman XP-65/F7F (1943/11)
  • The USAAF proposal had 2 x 37mm + 4 x 0.50; the USN had 4 x 20mm + 4 x 0.50
    • The USAAF version was not built
• North American F-86 & Derivatives
  • Baseline F-86: 6 x 0.50
  • FJ-2/-3/-4: 4 x 20mm
  • F-86F Gunval: 4 x 20mm

It wasn't absolutely perfect as the Typhoon had 12 x 0.303 replaced with 4 x 20mm not 6 x 20mm.

There is a book I own, that does just what you suggest, a series of like for like parameters.
It is the Bible for all airborne guns in WW2,and is used as the source document for many others, like this fighter book I also own.
A lot of math, too complicated to explain here, but the studies have been done, and both books are superb.

Dave

 

[David Fred]

I'm curious if there's a generalized metric when it comes to guns and cannon (allies at least)? I've generally seen figures similar to this

• 8 x 0.303, or...
• 6 x 0.50, or...
• 4 x 20mm, or...
• 2-3 x 30mm, or...
• 2 x 37mm/40mm...

It's clearly not perfect but the idea sort of came out of observations, as well as discussion with others.

• Hawker Hurricane (1935/11/6)
  • Designed with 8 x 0.303 to allow it to deliver enough bullets onto a bomber in 2-seconds
  • Later variants were developed with 4 x 20mm cannon
  • There was one variant built that had 2 x 40mm cannon
  • Exceptions: There was a variant built that had 12 x 0.303 and no cannon equivalent with 6 x 20mm
• Supermarine Spitfire (1936/3/5)
  • Designed with 8 x 0.303 for the same reason as the Hurricane
  • Proposals were made at some point for 6 x 0.50, and even something like 2 x 30-40mm cannon
  • Variants were built that carried 8 x 0.303, 2 x 20mm + 4 x 0.303; 4 x 20mm
    • Exceptions: The predecessors to the B-wing had only 2 x 20mm instead of 4 x 20mm, and the final set-ups had provision for 2 x 20mm + 2 x 0.50
• Bell P-39 (1938/4)
  • The P-39D had 4 x 0.303 in the outer wings, later replaced on some models with 2 x 0.50
• Vought F4U
  • 6 x 0.50 operational variants except one or two that had 4 x 20mm
• Bell YP-59 (1942/10/1)
  • One set-up had 1 x 37mm + 3 x 0.50; another had 2 x 37mm
• Grumman XP-65/F7F (1943/11)
  • The USAAF proposal had 2 x 37mm + 4 x 0.50; the USN had 4 x 20mm + 4 x 0.50
    • The USAAF version was not built
• North American F-86 & Derivatives
  • Baseline F-86: 6 x 0.50
  • FJ-2/-3/-4: 4 x 20mm
  • F-86F Gunval: 4 x 20mm

It wasn't absolutely perfect as the Typhoon had 12 x 0.303 replaced with 4 x 20mm not 6 x 20mm.

Here are just two of a LOT of tables from the guns book. If you are into aircraft guns, it's a mus have.

 

[GregP]

Post #23 makes me wonder if I am misreading the post. After the ammunition of the other four cannons is used up, there are only the two inner MG left, so there is no way they could use anything else.

Maybe I just misunderstand the wording.

Also, if 4 x 50s was optimum for WWII, then why is it that everyone raves about the Fw 190 with 20mm and 30mm connons and drool over the Me 262 armament? Weight of fire per second was a BIG factor in quick victories, and 4 x 50s wasn't going to make short work of any WWII fighter. But a hit from a big cannon with an explosive shell was going to turn a good day bad in a hurry.

I don't buy that 4 50s was optimum for anything except to fly at minimum weight. The P-47 had eight 560s and was devastating. The F7F Tigercat (after WWII in combat) was also a VERY hard hitter with 4 x 50s plus 4 x 20 mm. Getting in the way of THAT armament stream was a recipe for death.

 

[Glider]

For the mid war on I see the 4 x 0.5 as a bare minimum and I have seen official reports that follow similar lines. The USAAF wouldn't have gone for 6 x 0.5 if 4 was sufficient, The USN wouldn't have been so keen on getting the 20mm into action and the UK wouldn't have gone for 4 x 20mm as a standard. Even the IJAF went for 2 x 20 and 2 x HMG and the Italians went for 20mm and HMG when they finally upgraded.
I see nothing anywhere that says 4 x 0.5 is optimum

 

[Shortround6]

It rather depends on the opponent. Little or no armor and no (or rudimentary ) self sealing tanks?
It may depend on the pilot, how confident he was of his marksmanship?
I would also note that while a few (or mare than a few) F6F pilots managed to shoot down multiple opponents (or even make Ace in a day)in one flight the F6F could carry 400rpg which is about 30 seconds of firing time if all guns had the same amount of ammo or ammo boxes were full. That is a lot of gun time. There may have been a few pilots who ran into situations where they needed more firing time but I would wager that such instances were few.
I would note that the F8F with .50 cal guns had 325 rounds for the inboard guns and 300rpg for the outboard guns. They didn't expect it to need to fire as long?

 

[Glider]

It rather depends on the opponent. Little or no armor and no (or rudimentary ) self sealing tanks?
It may depend on the pilot, how confident he was of his marksmanship?
I would also note that while a few (or mare than a few) F6F pilots managed to shoot down multiple opponents (or even make Ace in a day)in one flight the F6F could carry 400rpg which is about 30 seconds of firing time if all guns had the same amount of ammo or ammo boxes were full. That is a lot of gun time. There may have been a few pilots who ran into situations where they needed more firing time but I would wager that such instances were few.
I would note that the F8F with .50 cal guns had 325 rounds for the inboard guns and 300rpg for the outboard guns. They didn't expect it to need to fire as long?

I disagree with most of this. If you base your view on little or no armour you might as well stick with 12 x 303 in a Hurricane IIb. Basing your view on the confidence of the pilot in his accuracy is also lacking logic because 99,99% of all pilots didn't have that confidence and I have never heard of any pilot taking guns out because they had such confidence if their ability. I give Galland as my example. A better shot you would be hard to find but when he flew the 109F he had extra guns added, he didn't take any out.
The F6F and the F8F were of course naval aircraft and the USN were very keen to fit their hellcats with 2 x 20 and 4 x HMG because they wanted the extra firepower and were not satisfied with 4 x HMG.

 

[Shortround6]

I think we are actually in agreement. I am not advocating the four .50 cal armement, simply commenting on the story that some US Navy pilots decided to fire only four of the their guns and keep the other two as reserve after firing for up to 20 seconds?

I don't know what the Navy was thinking when they specified the four guns in the F8F. Perhaps they were hoping to get the 1200rpm version which had been stuck in development for several years?
In any case the Navy lost little time in changing to 20mm cannon for the F8F-2.

I have no idea how widespread this practice was of shutting off two guns or what squadron commanders in general (or higher ups) thought of it.

 

[DarrenW]

Post #23 makes me wonder if I am misreading the post. After the ammunition of the other four cannons is used up, there are only the two inner MG left, so there is no way they could use anything else.

Maybe I just misunderstand the wording.

Hi Greg, I guess my post was a little ambiguous. I was discussing Hellcats that were equipped with six .50s, not a mixed battery of machine guns and cannon (the night-fighting Hellcat having two cannon, not four). My source for this is a VF-81 veteran who told me that he and others of his squadron would often fly with the two inner-most machine guns turned off, in order to conserve ammunition. He mentioned that in the heat of combat it was often easy to expend most if not all of your ammo, and pilots didn't want to end up defenseless on their trip back to the carrier.

 

[David Fred]

Post #23 makes me wonder if I am misreading the post. After the ammunition of the other four cannons is used up, there are only the two inner MG left, so there is no way they could use anything else.

Maybe I just misunderstand the wording.

Also, if 4 x 50s was optimum for WWII, then why is it that everyone raves about the Fw 190 with 20mm and 30mm connons and drool over the Me 262 armament? Weight of fire per second was a BIG factor in quick victories, and 4 x 50s wasn't going to make short work of any WWII fighter. But a hit from a big cannon with an explosive shell was going to turn a good day bad in a hurry.

I don't buy that 4 50s was optimum for anything except to fly at minimum weight. The P-47 had eight 560s and was devastating. The F7F Tigercat (after WWII in combat) was also a VERY hard hitter with 4 x 50s plus 4 x 20 mm. Getting in the way of THAT armament stream was a recipe for death.

The FM2 Wildcat carried four .50's but could carry 400rpg, as opposed to the 240 rpg in the F4F4 Wildcat. They didn't use it to save weight, but to carry more rrpg, more "trigger time". The F4F3 also carried 4 M2's, but 450 rpg. And the FM2 was a 2Q/43 introduction, later than F4F-4 from 2Q/42..

By the time you get to 3Q43, you get the F6F-3 Hellcat, Six .50 guns, but only 200 rpg, but the Hellcat had an "overload" capability, that increased load to 400 rpg.

So I think you see this debate going on between what is more important, and to which group of pilots?
Is how much lead can I put in the air with one burst, or how many bursts can I fire with onboard ammunition?

This book, contains all the specs for all the Marks/versions, but also some insight into the thinking.

 

[Kryten]

View attachment 495079

The FM2 Wildcat carried four .50's but could carry 400rpg, as opposed to the 240 rpg in the F4F4 Wildcat. They didn't use it to save weight, but to carry more rrpg, more "trigger time". The F4F3 also carried 4 M2's, but 450 rpg. And the FM2 was a 2Q/43 introduction, later than F4F-4 from 2Q/42..

By the time you get to 3Q43, you get the F6F-3 Hellcat, Six .50 guns, but only 200 rpg, but the Hellcat had an "overload" capability, that increased load to 400 rpg.

So I think you see this debate going on between what is more important, and to which group of pilots?
Is how much lead can I put in the air with one burst, or how many bursts can I fire with onboard ammunition?

This book, contains all the specs for all the Marks/versions, but also some insight into the thinking.

There is another consideration too Greg, and that's how long a burst can I fire to get my rounds on target.

I often read pilots reports of attacks being made with comments such as "opened fire at 400yds closing to 150" or "fired a six second burst", with the difficulty of air to air gunnery we may well be missing the possibility many pilots start firing and then adjust their aim onto target , that would be quite expensive in ammunition usage.

 

[David Fred]

There is another consideration too Greg, and that's how long a burst can I fire to get my rounds on target.

I often read pilots reports of attacks being made with comments such as "opened fire at 400yds closing to 150" or "fired a six second burst", with the difficulty of air to air gunnery we may well be missing the possibility many pilots start firing and then adjust their aim onto target , that would be quite expensive in ammunition usage.

You read that late in the war, last half of 1944 into 1945 (I'll find the book and page) that p-51's for sure had a little potentiometer behind the throttle, that electrically operated small trim tabs on the rudder fit very fine aim adjustments. No rudder pedal inputs, just the turn of a little knob.

And you're right, 240 rpg is 3-4 6 second bursts. Guys like Bud Anderson, a L-51 Double Ace says 2-3 second bursts, until on target, then use longer bursts,. 400 rpg gives you 40% more opportunity.

 

[Shortround6]

By the time you get to 3Q43, you get the F6F-3 Hellcat, Six .50 guns, but only 200 rpg, but the Hellcat had an "overload" capability, that increased load to 400 rpg.

So I think you see this debate going on between what is more important, and to which group of pilots?
Is how much lead can I put in the air with one burst, or how many bursts can I fire with onboard ammunition?

This book, contains all the specs for all the Marks/versions, but also some insight into the thinking.

The Navy had always been a bit disingenuous about the ammo capacity and fuel load of it's fighters, at least in the beginning of the war.
For instance the "official" ammo load for a "standard" F4F-3 was 300rpg and it was only supposed to carry 110 gallons of gas. Overload brought the gas to 147 gallons and the ammo up to 430 (or so) rounds per gun.
When carrying the pair of 100lb bombs two guns were supposed to be removed and ammo held to around 375 rounds per remaining gun.
How often this was adhered to practice I have no idea.

The detail Specifications for the F4U-1 show similar things, "standard" was 1200 rounds (200 per gun) and 178 gallons of fuel. Overload was 2350 rounds and 363 gallons of fuel (they filled the unprotected tanks inside the outer wings).

A question might be were these pilots who were saving ammo by shutting down two guns flying with either bombs or more than one drop tank and using less than full ammo to compensate?
P-47s could carry 425rpg when clean but often restricted the ammo load when carrying under wing stores.
F6F manual does show an ammunition counter in the instrument panel but in the heat of battle the pilot may be keeping his head out of the cockpit.

 

[David Fred]

The Navy had always been a bit disingenuous about the ammo capacity and fuel load of it's fighters, at least in the beginning of the war.
For instance the "official" ammo load for a "standard" F4F-3 was 300rpg and it was only supposed to carry 110 gallons of gas. Overload brought the gas to 147 gallons and the ammo up to 430 (or so) rounds per gun.
When carrying the pair of 100lb bombs two guns were supposed to be removed and ammo held to around 375 rounds per remaining gun.
How often this was adhered to practice I have no idea.

The detail Specifications for the F4U-1 show similar things, "standard" was 1200 rounds (200 per gun) and 178 gallons of fuel. Overload was 2350 rounds and 363 gallons of fuel (they filled the unprotected tanks inside the outer wings).

A question might be were these pilots who were saving ammo by shutting down two guns flying with either bombs or more than one drop tank and using less than full ammo to compensate?
P-47s could carry 425rpg when clean but often restricted the ammo load when carrying under wing stores.
F6F manual does show an ammunition counter in the instrument panel but in the heat of battle the pilot may be keeping his head out of the cockpit.

The book I have, as shown in my last post show the following.
F4U-1 6x .50 M2 Browning, 400 rpg standard load (1Q'43)
F4U-1A SAME (3Q'43) Addition Of 6mm armor above/rear pilot seat in canopy
F4U-1C 4x 20mm Hispano 231 rpg (3Q'43)
The -1D (3Q'44) and -4 (1Q'45) versions are back to 6 .50 @ 400 rpg.
My book agrees with your P-47 numbers, with 267rpg being standard load.

 

[DarrenW]

A question might be were these pilots who were saving ammo by shutting down two guns flying with either bombs or more than one drop tank and using less than full ammo to compensate?

You may be on to something Shortround6. The Hellcat pilot I spoke with also commented that they carried either bombs or rockets but never both on the same mission. Likewise, a reduced ammunition load (200 rather than 400 rpg) may have been a matter of keeping to the aircraft's "clean" operational weight as much as possible (an/or lighten the loading on the wings) so performance didn't suffer too greatly while carrying external stores. In this situation it would be smarter to conserve ammunition whenever possible.

Someone who was an aircraft armorer at one time might know the answer to this question....

 

[Shortround6]

It might depend on the carrier they were operating from, weather conditions and so on. It sounds like they were were doing ground support.

According the charts here http://www.alternatewars.com/SAC/F6F-3_Hellcat_ACP_-_1_October_1945.pdf

They do list full ammo but an F6F-3 with drop tank and rockets needs 622 ft to take off with a 15kt head wind compared to 519ft when carrying only the drop tank.
At 25 kts the distances are 452ft and 372ft. This may be for "standard conditions and hotter than "standard" may require more deck or lighter aircraft?
The six rockets and launchers appear to add 864lbs. 200rpg of .50 cal ammo for six guns is 360lbs so it is a useful offset.

I could be totally wrong but it does seem to be a possible explanation. 200rpg gives about 15 seconds of firing time for ground strafing which is way more than one pass. Keeping two guns out of action may not hurt much and provide that feeling of comfort for the trip home?

 

[Washing Machine Charlie]

If I'm not mistaken, you can hear the guns alternating fire sequence, in the video.

The 40mm cannon on the Hurri MkIID were actually configured to fire alternately rather than both at the same time, presumably to reduce the impact of recoil.

Here's a cool vid showing it in action: