Guns & Cannon: Rules of Thumb

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. Several Wildcat guys kept the two outboards for get-me-home insurance. Swede Vejtasa and George Wrenn for certain; sorta recall Joe Foss saying he occasionally did because of some marginal ammo quality.

Long ago I ran the numbers: the 50% increase in firepower (four .50s to six) only produced a 10% increase in lethality (destroyed credits as a ratio of total destroyed-probable-damaged claims.) For WW II, four .50s was optimum.

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. Several Wildcat guys kept the two outboards for get-me-home insurance. Swede Vejtasa and George Wrenn for certain; sorta recall Joe Foss saying he occasionally did because of some marginal ammo quality.

Long ago I ran the numbers: the 50% increase in firepower (four .50s to six) only produced a 10% increase in lethality (destroyed credits as a ratio of total destroyed-probable-damaged claims.) For WW II, four .50s was optimum.

I recently talked to a Hellcat pilot who said that it was practically squadron policy to only use the two inboard .50s after the ammunition of the other four guns was exhausted. Like Barret said, for insurance that you still had something to protect yourself with when you least expected it.

I tried posting earlier this morning--seemed to go thru but now it's vanished. So here's The Cliff Notes Version:

Yes, the F4F-4 was upgunned at RN request v. the Germans and Italians. Combined with the folding wing, the extra weight degraded performance because there was no HP increase.

Long ago I ran the numbers: in six-gun .50 cal. fighters, the 50% increase from four only resulted in a 10-11 % increase in lethality, computed by comparing Destroyed credits among destroyed/probables/damaged. (Jimmy Thach famously said of the P-47, "A pilot who misses with four guns will miss with eight.") So, IMO the optimum WW II VF armament was four Ma Deuces, which usually permitted more trigger time than six. Some F4F aces I knew (Foss, Vejtasa, Wrenn, etc) kept the outboards as get-me-home insurance.

Agree that four .50s was sufficient. AAF and Navy were moving to 4 with the P-51A/B/C and the FM2 and Bearcat. They were all harmonized to a spot in the distance, so if you missed with 4 you would miss with 6. And save about 200# per gun with ammunition, so a 400# savings for two would really help as the F4F4 pilots lamented.

Barrett said:

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. Several Wildcat guys kept the two outboards for get-me-home insurance. Swede Vejtasa and George Wrenn for certain; sorta recall Joe Foss saying he occasionally did because of some marginal ammo quality.

Long ago I ran the numbers: the 50% increase in firepower (four .50s to six) only produced a 10% increase in lethality (destroyed credits as a ratio of total destroyed-probable-damaged claims.) For WW II, four .50s was optimum.

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P-39 Expert said:

Agree that four .50s was sufficient. AAF and Navy were moving to 4 with the P-51A/B/C and the FM2 and Bearcat. They were all harmonized to a spot in the distance, so if you missed with 4 you would miss with 6. And save about 200# per gun with ammunition, so a 400# savings for two would really help as the F4F4 pilots lamented.

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Sometimes it comes out as a surprise that countries other than USA actually produced aircraft to suit their needs. You know, to shoot down other people's aircraft that were big & strong lumps of metal in quite a number of instances, while some unsporty enemies even installed armor and other protection on their A/C.
Wonder whether it was because it past more than 70 years after the ww2 ended?

P-39 Expert said:

Agree that four .50s was sufficient. AAF and Navy were moving to 4 with the P-51A/B/C and the FM2 and Bearcat. They were all harmonized to a spot in the distance, so if you missed with 4 you would miss with 6. And save about 200# per gun with ammunition, so a 400# savings for two would really help as the F4F4 pilots lamented.

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And you miss with all of them if the bloody guns don't work properly, as was the case in ALL 50cal wing installations prior to the latter half of 1942! What was the RAF supposed to do in the preceding 3 years that it had been fighting? Throw rocks at the enemy and hope for the best?

Yes, 4x50cals was a good solution from about November 1942 onwards but it was NOT a viable solution earlier than that. Why is this so hard for people to accept?

People keep ignoring the facts 1. that in 1940 (or a good part of it) the M2 .50 cal gun was a 600rpm gun, at some point in 1940 it was modified into the 800rpm version so any planes ordered in 1940 (and delivered in 1941) have to take that into account. Please note this is for free firing guns (not fuselage synchronised guns which were even worse.) Deliveries of the higher cycle rate guns and parts kits to modify the old guns go to who first?
6 guns times 600rpm = 60 shots per second. 4 guns times 800rpm (13.3rps) is 53.3 rounds per second. Four of the old guns is 40 rounds per second.
2. British were ordering .50 cal M1 Ball ammo and equivalent AP, incendiary and tracer ammunition in 1940. The M1 series of ammo had muzzle velocities of around 2500fps. The US had changed to the M2 series of ammo in the very late 30s with MV of 2800-2900fps (depends on exact bullet and source) so the ammo was going to be a bit less powerful than the ammo used by the Americans in 1941 and beyond.
3. By very late 1942 or early 1943 the US was standardizing on the M8 AP incendiary round which would comprise well over 90% of the ammo in a fighters belts.
Earlier ammo loads comprised mixed belts of ball*, AP, incendiary and tracer.

*the only difference in US .50 cal ammo between AP and ball is the steel core is hardened in the AP.
SO the 4 gun Mustangs and the FM-2 Wildcats use faster firing guns with higher velocity ammo with dual purpose ammo compared to the 1940 guns and ammo the British might have been expecting.
I would note that the US also had a long and torturous affair with trying to develop and 1000-1200rpm version of the .50 cal gun which started at least as early as 1942 and involved at least three different companies and multiple projects in each company and only finally paid off in the winter/spring of of 1945 with a batch of guns that had a "T" number and were later standardised as the M3 .50 cal gun. Very late war fighters (like the F8F) have to viewed in that light, wither or not they actually got the M3 guns.

Edit. I may be in error on the availability of incendiary ammunition in 1940 for the .50 cal gun. Sources vary but the Incendiary ammo may date from after 1940.
The US has a rather confusing system of nomenclature in which the first model of almost anything is the M1 regardless if it is a rifle, tank, cartridge or..............
I would note that a lot of the information on the .50 is a bit confusing as
.50 Caliber Browning (12.7 x 99 mm) Ammunition
has different velocities for the the M2 Ap and the M33 ball (they really went through 31 designs of ball ammo?) even though they use the same weight bullet and the same powder (pressure is slightly different but quantity of powder and pressure was flexible as long as desired velocity was achieved.)

I'm with you 100% SR. This tendency to look backwards and view the early part of the War from the perspective of 1944 distorts reality to an absurd degree and ignores all the hard-won lessons, often paid in blood, that enabled the progress you describe.

I would also like to point out that both the US and British were looking at and specifying much heavier armament in late 1939 and through 1940 than either eight .303s or four .50s.
The Curtiss XP-46 project started in Sept of 1939. Two .50s in cowl like the early P-40s but up to eight .30 cal guns in the wings.
The NA Mustang. Two .50s in the fuselage, a .50 in each wing and two .303s in each wing.
The Curtiss P-53 started work in 1940 with up to eight guns in the wings.
The P-47 was specified with either 6 or 8 .50 cal in the wings.
By the time you get to the Curtiss XP-60 (XP-53 with new engine) you have 275 sq ft laminar flow wing, in part to house eight .50 cal guns. Plese note these programs overlapped to a considerable extent. The XP-60 actually flying 11 days before the 2nd prototype (fully equipped) XP-46 in Sept of 1941.
British were going for either twelve .303s or four 20mm cannon, with cannon being prefered as soon as available.

It took a lot of combat experience to show that the four .50s were actually sufficient against small/medium fighters and small bombers.

As a further note, the Americans and British and Germans were fitting self sealing tanks, armor and BP glass to exiting aircraft at an almost frantic rate in 1940.
Expecting the Japanese to not follow suit would require a crystal ball of the highest quality

When specifying or ordering aircraft you are hoping the first planes can go into service in a little over a year to two years, planes already developed/in porduction can take a bit less time but remember, The French ordered 100 Wildcats in Oct 1939, The British took over the order upon the fall of France and the FIRST French ordered aircraft is accepted July 27 1940. It takes until Oct 31st 1940 to take delivery of 81 Martlets.
What armement should the British have specified in late 1940 to be fitted to aircraft that might not show up in England until late 1941 or 1942?
First Wildcat kill was supposed to have been a JU-88 on Dec 25th 1940. Later British Wildcat kills were FW 200 Condors. Sticking with four guns and hoping the Germans don't replace the FW 200 converted airliners with real 4 engine military planes might not be considered the best planning.

Does anyone have a timeline of sorts for US .5-in ammunition? Looking at the references to incendiary ammunition - I was reminded of a paragraph from an undated British document 'Observations on Japanese Air Activities in Burma': Absence of incendiary ammunition for the .5 calibre guns had been responsible for many E/A getting away. This is the constant complaint of the A.V.G. pilots.

Was this a supply issue or was the .5-in M1 incendiary not in general use until a later date?

The FAA asked for 6 HMGs in the Martlet as their task was to destroy Ju88 and He111 bombers and torpedo bombers, Combating other fighters in the strike escort role was a secondary task. They were still using them in the secondary role to escort FAA strike aircraft in Norway in 1945 against Me109Gs. The USN may have had different priorities in their planning for the use of F4Fs.

Greyman said:

Does anyone have a timeline of sorts for US .5-in ammunition? Looking at the references to incendiary ammunition - I was reminded of a paragraph from an undated British document 'Observations on Japanese Air Activities in Burma': Absence of incendiary ammunition for the .5 calibre guns had been responsible for many E/A getting away. This is the constant complaint of the A.V.G. pilots.

Was this a supply issue or was the .5-in M1 incendiary not in general use until a later date?

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For the British it seems as if there was no .50 incendiary ammunition in use until 1942.
See; .50 inch Browning Incendiary - British Military Small Arms Ammo

Where/how the AVG traced their supply line I don't know. When the US adopted the M1 incendiary bullet may be good question. There doesn't appear to be an M2 incendiary which probably means there was no incendiary ammunition to go along with the ball ammo in the 1930s.

I have found this

"The first round that had an official designation though was the M1 boat-tailed .50 in 1931. But still the military had higher expectations for its fifty caliber round. Basically any improvements to the .50 caliber round were unremarkable until 1937, due mostly in part to the lack of funding for the project because of the Great Depression. In 1937 though the fifty-caliber ammunition underwent various propellant improvements which resulted in the velocities up to 2,700fps. Further testing in the 1940's by reducing the projectile weight from 750 grain to 710 grain increased the velocity to an incredible 2,810,with a chamber pressure of 55,000 psi and it was capable of penetrating an astonishing 3/4 of an inch of armor plate at 600 yards. The M1 round was now replaced in 1941 with the newer M2 version."

From http://www.smallarmsreview.com/display.article.cfm?idarticles=3055

Which puts the M2 ball and AP later than I thought and really helps explain the British preference for the 20mm. I would note the velocity for the .50cal is all over the place with anywhere from 2810 to 2900fps being quoted for the M2 ammunition.

The German's were designing the Bf 109 with just 4xLMG's, until they discovered the RAF was planning to introduce a 8x LMG's on their next generation of fighters

redcoat said:

The German's were designing the Bf 109 with just 4xLMG's, until they discovered the RAF was planning to introduce a 8x LMG's on their next generation of fighters

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(my bold)
With just 2 LMG...

I think even the four machine guns were a modification.

Some sources (in English) say either 2 two machine guns or a motorcannon were the original intended armament. Please remember that original engine was an early Jumo 210 (and prototype flew with a Kestrel because the Jumo wasn't ready) and there is only so much you can do with around 600hp. The Cannon was nowhere near ready either and with the British going for eight guns the 109 was looking a bit out of it. They tried a 3rd machinegun through the prop but reliability was iffy which forced the extra gun/s out into the wings.

Alright - digging through some old documents here ...


Found a British order (6 May 1940) for American .5-inch Brownings and ammunition; AP (742,500), Ball (1,552,500), Incendiary (405,000), Tracer (8,100).

Another British paper (23 Oct 1940) Notes on the Position of American Calibre .30 and .50 Ammunition:

'Ready for production'. Ready for production for the RAF? ... or the bullet wasn't in widespread production yet period. Not sure. Or maybe this was noted in contrast to the British .50 'de Wilde' incendiary - which was just in the design stage at this point. I see in all British documentation there is no mention of an 'M1' Incendiary - unlike the other three types of ammunition. Another hint that the US design wasn't in production or somewhat experimental at this stage.

From a British document (24 Oct 1940) referenced firing trials of US ammunition: .3" and .5" Incendiary and Tracer: We have fired these at Orfordness and find them of little value as igniters of self-sealing petrol tanks. The U.S. report quoted them as effective in ignition of unsealed petrol tanks, but since the German Air Force up to the present has not used unsealed petrol tanks this claim is not of importance to the R.A.F.

I have a feeling the first real American production M1 .5-inch incendiary was developed parallel to the British Mk.I - i.e. both based on the 'de Wilde' design, and both not available until 1942. There was an incendiary design in the US before then - but perhaps not in service.

Notes from a Ministry of Supply meeting (25 Jun 1941): Incendiary: -- A design is available now and an improved design should be ready within a few weeks which is suitable for production in U.K. -- In the U.S.A. trials are proceeding with a similar design suitable for U.S. production. -- We are ahead of U.S.A. and can go into production at once.

Thank you !

Ok - I think I was right. Looking at an older thread we were posting in - you posted a diagram of the M1 Incendiary:



That is absolutely Dixon's 'de Wilde' design, and not the 1940 'copper cup' design described.