Which Gun?

by Jim "Twitch" Tittle

Article Type: Military History
Article Date: October 30, 2002

Which Do You Choose?

Adequately Armed?

An enemy fighter is coming straight for you and you can choose twenty-five rounds from a .50 caliber or 20mm. Which do you choose?

Of course it’s not that simple. While we can’t decide for you, we will show some comparisons in a more ballistic sense. But we must never forget the human factor. A pilot with lesser guns who is accomplished in aerial gunnery is more deadly than one who hopes to spray and pray with heavy armament.

Early On

Often times fighter design left physical limitations as to the largest caliber gun that could be mounted in a given location. The Bf 109 had 7.9mm machine guns (MGs) in the upper cowl that were upped to 13mm later. 20mm would not have fit.

In the 1930s rifle caliber (.30 and metric equivalents) guns and ammo seemed appropriate for any perceived threats and many aircraft designs took those weapons into account. Germany was about the only country seriously developing cannon armament for future aircraft. As we know this escalated to heavy multi-cannon armament by the end of the war. While rifle caliber ammo could pierce the skins they lacked severe damage power that cannons had with explosive shells.

Rate of fire (ROF) and rounds per minute (RPM) were better with early MGs but was equaled in later model cannons. So if a gun could fire at the same rate it was better to have the heavier bore, right? Well, yes and no. A 20mm round took more space than a .50 caliber round so you had fewer rounds to shoot. So you had to be a good shot and frugal with ammo.

Japanese and German pilots were rigorously trained early on but later pilots were thrown into combat with much less experience. These countries cadre honed their shooting skills in China and the Spanish Civil War. When many were gone their replacements were ill suited to take on Allied pilots who received training that was “very good” but not “excellent.” The theory that many good pilots could defeat fewer excellent ones was the thinking, regardless of type of plane flown. Hmm?

Sure, there were aces like Saburo Sakai who took down a P-39 with four 20mm rounds and could come home with triple kills with only 60 RPG in their two wing-mounted 20mm cannons and cowl-mounted 7.7mms. Hans Marseille could do the same with one 20mm. But by and large they were the exception rather than the rule by the time the Allies got up combat steam. The British thought more like the Continental countries in arming with 20mms and .303s. American airmen generally were content with .50s and it is a proven fact that they could shoot down enemy aircraft with them. But American P-38 pilots and late-war Corsair and Hellcat night fighter pilots, who had experience with 20mms, liked them for their power.

Six In The Wings

There are more factors involved than simply who had the biggest bore guns. Protective armor was a critical factor. Japan’s concept early on was to have none so as not to impede maneuverability. Self-sealing fuel tanks were another item that added weight and complexity. But these things were pilot lifesavers. You can’t evade bullets forever and a mid-war Hellcat F6F-3 that fired a 2-second burst from its six Browning M2s put about 144 rounds of ammo down range with 2,256 more left! If just one hit a Zero’s fuel cell is was all over. By comparison, the A6M3 Zero with two Type 99-2 20mm cannons and two Type 97 7.7mm MGs put thirty-five 20mms and fifty 7.7s out in the same two seconds. 7.7s generally bounced off heavy American armor and thirty-five 20mms are easier by percentage of numbers to miss if you are in the sights. One 20mm could do serious damage but a sturdy airplane lessened the probability. The jacketed .30 cal./7.7mm/7.9mm could penetrate canopies and softer parts of the skin, however, and they assisted kills.

German aircraft design had a very good blend of factors—they generally had substantial weaponry, armor and self-sealing tanks plus were small and gracile enough to respond well to control input.

What To Compare

In the Battle of Britain a Bf 109E with two MG FF 20mms put out the same number of rounds as two Type 97s: thirty-five in two seconds and the two MG 17 7.9mm MGs spit out about seventy-three projectiles. An eight-gunned Spitfire I or II and the Hurricane I sent 304 rounds of puny .303 out in two seconds from the Browning M2s but none of the rounds were explosive.

If we compare weight of fire in kilograms in that two seconds it looks like this:

• F6F-3 = 7.28kg
• A6M3 = 4.96kg
• Bf 109E = 4.74kg
• Spit and Hurri = 3.44kg.

If the weight and rate of fire are translated into an energy format, like kilowatts per second, it shakes out like this:

• F6F-3 = 1,370kw
• A6M3 = 520kw
• Bf 109E = 530kw
• Spit I & Hurricane I = 480kw.

All other six-gunned American fighters would equal the Hellcat in firepower though the eight-gun P-47 could manage 1,830 kw of power per second.

And we can fool ourselves too since all 20mms were not equal even if ROF and RPM were the same. Projectile weight varied also. An MG/151 used 92 and 115-gram shells while the Hispano Mk II used 130-gram ammo. We must factor in projectile speed in feet per second (FPS) or meters per second (MPS) measured at the muzzle. With the Hispano firing 130-gram rounds at about 880 MPS or 2,887 FPS compared to the MG/151 with 92-gram rounds going 800 MPS or 2,625 FPS. Our Type 99-2 had a large shell at 129 grams but fired at slower RPM and velocity was slow at 625 MPS or 2,050 FPS. The lighter round goes faster but expends its energy sooner and has a somewhat lessened impact if it hits.

A split second in the arrival time of ordnance to your plane can mean the difference of life and death and inches missed may have well have been miles since the effect was the same—no damage.

The Spitfire VIII With 4- 20mms

Total firing time is not to be overlooked either. If you had potent weaponry and very a short total firing time till you expend all your ammo this was not healthy in a combat situation especially if you were not an expert aerial marksman. As an example, a Spitfire VC or VIII with four 20mms and 120 rounds per gun (RPG) fired the Hispanos at a rate of about 10 RPS per gun—or twelve seconds of fire-—where the FW 190A-8 with 200 outboard/250 inboard RPG pumping out some 12.5 RPS of 20mm still had a longer firing time. Even when the outboard guns’ 20mms ran dry the Luftwaffe pilot still had the inboard pair and the 13mms on the cowl to shoot with a total of over twenty seconds total firing time. That’s slightly unfair, since the A-8 was a later model, but you get the idea.

The P-47 pilot with 425 RPG had 34 seconds of firing time from its 3,400 rounds and the even the lowly P-51 had about twenty-two seconds from the fifties with 270 RPG till four ran dry and another ten seconds from the inboard pair when the 400 RPG ran out.

Every ordnance chief had a variety of types of rounds also. Whether .50 caliber or 20 millimeter, they were mixed on the ammo belts. Armor piercing, incendiary, tracer and explosive shells alternated in combinations that suited the plane, pilot or mission best.

Bf 109G Cowl Bumps Accommodated 13mm Breaches

And certainly later models with all sorts of mixed and add-on weaponry toggled the factors involved. A Bf 109G with one 20mm and two 13mms was not as effective in doing damage as when it had two additional 20s in the under-wing gondolas but they hampered performance. The Bf 109K-6 with its three Mk 108 30mms and two 13mms had double the kinetic energy of the K-4 with only one 30mm and two 13mms.

The Me 262 is the winner of WWII considering its two 30mm Mk 108s and two Mk 103s with 1,590kw output. Its firepower was more than the Bf 109K-6. Of course this doesn’t begin to tell the tale in real world use. Either plane’s 30mms could wreak havoc in Allied bomber formations. Three shells were deadly enough to bring down a B-17. The 108 weighed 312 grams and the 103 weighed a whopping 330 grams! The heavier 103 was actually much faster but the weapon fired slower.

All In The Nose

This escalation begins with a 10-11 gram rifle caliber at a .30 caliber average through the Japanese Type 2 and Type 3 12.7mm/.50 caliber at 34 and 52-grams each, 13mm MG/131 at 35-grams and the .50 M2 plus the 12.7mm Russian UBK at about 48-grams. The Russians had good weapons in their 23mm Vya and NS-23s offering 200-gram rounds. The VYa cyclic rate was slower than other 20mm weapons but is was a speedy round 960 MPS or 3,149 FPS! Though the NS-23 was much slower.

Korea brought some of the same basic weapons, though higher firing rates, and we see an extension of WWII technology. The MiG 15 had a much higher weight of fire in its two-second burst than an F-86 with 17.38kg and 11.64kg respectively. Firing time for the F-86’s six M-3 MGs was about fifteen seconds compared to ten for the MiG 15 with its two NS-23 23mm and one NS-37 37mm cannon.

Put The Guns Where?

Both the F-86 and MiG 15 mounted their weapons in the nose for fuselage axis bore sighting. This has been the mark of jets since the Gloster Meteor and the Me 262: guns in the nose where the V-12 used to be. Since WWI, firing through the propeller disc has needed extra measures like interrupter gears to synchronize the bullet firing between the passings of the prop blades. And the Daimler-Benz engines had special hollow block sections to accommodate the cannon in 109s.

In WWI we see a mixed bag of practice in the “where to mount guns” question. Twin-engine planes had it easy. P-38s, Me 110s, 210s, 410s, Mosquitos plus many more enjoyed the “sight to infinity” line of fire. The other method was employed on most single-engine planes—guns that fired from the wings outside the prop arc. This necessitated a convergence point somewhere down range that varied depending on the weapons, plane and personal pilot preference. Through experience they had arming crews set them anywhere from 300 yards down to 150. Some even staggered the distances in pairs. The five gun Bf 109s used all three ways to mount—through the engine and prop hub, through the prop and outside the prop arc.

RAF Whirlwind’s Easy Access 20mms

Wherever the convergence was in wing mounts the guns would put about 90 percent of their projectiles in a box about three feet square for the most devastating effect. At lesser distances less ammo would be in on target. American pilots often speak of getting so close so as to use rudder to skid and point one wing’s set of guns at the enemy. Fuselage mounted guns, like the Bf 109 models without the wing guns, put all the ordnance straight out in front all the time. This may be why 109 pilots who were good shooters did even better since the effect of fire was equal at all ranges. This is not to say that converging wing weapons did not score at long range. Mustang, Hellcat and Thunderbolt pilots did get kills at 1,500 feet and even rarely at half a mile.

Deflection shooting was easier with fuselage mounts too. But no matter what you were shooting a large number of factors had to come together. Any ordnance, if it struck at certain angles and places, could deflect and/or explode outside the aircraft. The rounds that hit squarely and detonated inside the plane were the deadly ones. Armored sections assisted in this defense greatly.

Is There A Real Winner?

Well If I were to choose what I’d want to be armed with, it would be similar to the FW 190A-8’s two 13mm MG/151s and four 20mm MG/15s. This combination would give me 14kg of ordnance weight in a two second burst and about 1700kw of energy per second. In the A-8 it provided sixteen seconds with all guns firing, twenty seconds with the inboard 20mm and the 13mm MGs and twenty-four with the last rounds of just the 13mm firing. The 92-gram 20mm shell was almost as fast as the Browning .50 and fired at nearly the same rate, while the 115-gram shell was slower ballisticly. Yes, the MG/151’s speed was less than the Hispano 20mm’s.

But considering the packages they were placed in, I’d rather have the extra rounds the 190A-8 carried than that of the British fighters which usually carried no more than 150 RPG and no 13mm/.50 calibers though the Tempest could output 2,300kw of energy per second from its four Mk V 20mms with 130-gram projectiles. And though the FW’s cannons were in the wings the 13mm MGs were in the upper cowl for infinite sight-ability.

There are simply too many variables to decide which aerial gun and how it was mounted was “best.” Best was usually what you had and you made the best of it until the techno-types caught up with real-world experience. In hindsight we have an easy job of picking and choosing with all the specifications in some respects, but simple, overall specs do not tell the tale of ballistics in the real world of aerial combat.

I’ve studied knock-down power in many different weapons for years but I like the unfettered way this web site does it for aircraft. The books and magazine articles I’ve studied can get too esoteric and factor in too many complicated equations at times, I believe. Simple is better and you’ll find a lot here.

Sources

• Caiden, Martin
  Me 109
  Ballantine Books, NY, 1968
  
  
• Francillon, Rene J.
  American Fighters of WWII Vol. 1
  Doubleday & Co. NY, 1968
  
  
• Green, William
  Fighters Vols. 1-2-3-4
  Doubleday& Co., NY, 1960-62
  
  
• Mason, Francis
  RAF Fighters of WWII Vol. 1
  Doubleday & Co. NY, 1969
  
  
• Sharpe, Michael, Scutts, Jerry & March, Dan
  Aircraft of World War II
  PRC Publishing Ltd, London, 1999
  
  
• Sweetman, John
  Fighter Weapons
  Ballantine Books, NY, 1969
  
  
• Windrow, Martin
  German Fighter of WWII Vol. 1
  Doubleday & Co. NY, 1968

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