20mm cannon vs Machine gun armament debate:
The debate between the .50cal and 20mm cannon for fighter aircraft armament has gone on for decades and we won't solve it here. However, I think it is interesting to look at other factors which drove the decision rather than the raw performance numbers of the guns.
US production and armament: The US had the .50cal in production since WWI and in 1939 when most of the US fighters were being designed for WWII the .50cal was adequate and better than the .30cal. In 1940 the US Army directed manufacturers to mount six guns if they could.
The first P-40 model had 4x .30cal in the wings and 2x .50cal in the nose. The P-39 had 2x .50cal in the nose and 2x .30cal in each wing and a HS 20mm or 37mm gun firing through the nose. The Wildcat had 4x .50cal in the wings.
The US standardized the .50cal as it was proven, reliable, easy to manufacture, and carried more ammo. While the 20mm HE round was vastly superior in one-hit destructive power, the .50 caliber setup traded individual explosive punch for a higher rate of fire, much greater ammunition capacity, lighter gun weight, and higher reliability. In air combat it's difficult to get a hit so the more ammo you throw out the better the hit chance which is what you need with new pilots. An experienced pilot might trade the outer guns for less weight and maneuverability.
Later in the war pilots requested more firepower against armored German fighters and the P-51D wing was redesigned for 3 in each wing. They could have fitted 20mm but the ammo per gun would drop from 250-400 rounds per .50cal gun to 125 round for the 20mm. That's not near enough ammo when performing a bomber escort being in the air for 4+ hours or patrolling in the Pacific.
The P-51B/C: They had 4x .50cal. Why not 6? Supposedly the wing was too thin and had a fuel tank and landing gear in the wing so not enough room, they probably could not fit a 20mm in the wing either. Weight was also a concern for high altitude and long range performance. So the .50cal was about the only choice. The 6 guns firing up 60-70 rounds per second gave a greater chance of hitting a maneuvering target than 4 guns . Early war ammo loadout was 2x AP, 2x API and 1x Tracer. Late war and for strafing was 4x API-T (more effective) and 1x API or AP.
Mustang Wing Design Problem: The Apache had 4x 20mm but had problems fitting them in the wing and went to .50cal. The P-51B had 4x .50cal but the wing was so thin (no room for an extra one) the guns had to be canted, which resulted in jamming, especially in high G turns. The later models had a thickened wing eliminating the problem and modified to allow an extra gun which pilots requested because later war German planes were better armored. Could the P-51D Mustang handle 4x20mm HS? Evidently yes.
Some Mustang P-51D pilots had the center gun removed to make room for additional ammo and greater firing time during long escorts.
Sustained Firepower: The standard six-gun configuration would deplete the center and outboard guns in about 20 seconds of continuous firing, leaving only the inboard guns active. Removing the center gun allowed the outer ammunition trays to hold 500 rounds, ensuring all four remaining guns would fire uniformly for a longer duration.
Removing two heavy Browning M2 machine guns and their respective mounts shaved significant weight off the wings, which improved roll rates and slightly boosted fuel efficiency.
Wing Gun Convergence/Harmonization Problem: Because the guns are not nose mounted, the "Sweet Spot" will be at a specific range. Closer or further away from the Sweet Spot means fewer rounds on target. Wing guns could be aligned to give a "Buzz Saw" effect or a wider Pattern Harmonization by staggering each gun to a specific range like 200, 250 and 300 yards rather than one range for all guns. The "Sweet Spot" that can concentrate all guns for a "Buzz Saw" type effect was from 200 to 400 yards which could be determine by the pilot or the type of engagement he is expecting. At point blank range you had to offset your aim to hit with only guns from one wing to ensure hitting the fuselage. Nose mounted guns have the advantage of concentrating their firepower out to 600+ yards.
Here is a diagram of the issue:
Should this problem be included in gun fire rules in a game by modifying firepower? Are there any rules that do and how? I think it would be an interesting Risk-Reward Decisions for players to set it before the game.
Going by firepower/weight without considering harmonization gives wing mounted guns a non-historical advantage over nose mounted guns. Six wing mounted guns may not put more rounds into the target than three nose mounted guns.
The US had problems with the HS 20mm cannon. They had a choice of fixing it or going with a proven .50cal. The US was fighting on two fronts and the Jap planes did not need 20mm cannons to bring them down. I think just based on that there was not a lot of emphasis to get the 20mm HS fully functional in fighter wings. Changing manufacturing lines, stopping production of one type of ammo for another, retrofitting aircraft already deployed and additional logistical issues was not worth the effort.
Not all 20mm guns are equal. The wing mounted MG-FF in the Bf-109E and FW-190A had a 60 round drum, low velocity and very poor HE round. The 151/20 mounted in the nose was much more effective. The Jap 20mm gave poor performance than the HS and German 20mm. The Russian ShVAK 20mm was not as effective as the German 20mm. The American HS 20mm had performance problems because of manufacturing headspace requirements. The British HS 20mm worked pretty well but had a problem mounting it in a thin wing until the end of the war. From what I can find only the Bf-109E and I-16 had wing mounted 20mm guns before 1941 (fighters).
The German 151/20 had the most effective explosive shell. The HS 20mm MV was about 850 m/s and the German 20mm was about 700 m/s. The German gun was more deadly but the HS more accurate at longer ranges. The ShVAK was 750 m/s and had the least effective HE shell.
Mustang Mk1A problems: The 4x American MS 20mm wing mounted cannons faced reliability problems. The layout gave constant problems because the guns were mounted 30 degrees had the belts twist, bind or tear under high G maneuvering after only a few rounds. The British had to strip the guns and feeding mechanisms and install their HS 20mm.
Hurricane 20mm problems: Had problems mounting 20mm gun in the wing. They tried underwing pods which failed. They eventually re-engineered the wing to an all metal double spar with cutouts to mount the gun (Hurricane II). When all four guns were fired the deceleration was noticeable. It used a 90 round drum prone to jamming giving no more than 10 seconds firing time. It was strictly a ground attack aircraft.
Spitfire 20mm problems: The Mk Ib the gun had to be mounted sideways which gave jamming problems. The thin wing and single spar of the Spitfire flexed during hard turns throwing the guns out of alignment. The wing needed bulges to fit the ammo drum. It was so unreliable that some units wanted the .303s back. The Mk V appears to be the first model to deploy the HS gun with a belt feed which eliminated the jamming problem.
A Wing: 8x .303 guns
B Wing: 1x HS 20mm and 4x .303 guns in each wing mounted sideways with a 60 round drum. Prone to jamming.
C Wing (Universal): Extensively redesigned to fit HS cannon and a 120 round belt fed mechanism which eliminated the jamming problem. 4x HS 20mm or 2x HS 20mm and 4x .303 guns.
E Wing (Ground Attack): 2x HS 20mm belt 120 rounds and 2x .50cal with 250 rounds.
The B & C wings guns had the same muzzle velocities.
Zero 20mm problems: The Type 99 Mk I had a 60 round drum with a spring which could fail to feed rounds into the breech under heavy G. Later versions had the Type 99 Mk II had better performance with belt fed 125 rounds per gun. The ballistics of the 20mm and 7.7 machine guns was different resulting in decreased effectiveness when both were fired.
Bf-109 problems: It had 1x MG-FF 20mm in the wing with a 60 round drum (7 seconds of firing) with a RoF of about 530 rounds/minute. The wheel retracting mechanism was in the fuselage, not the wing so it had the room to fit. It was removed in the F and G models for better maneuverability.
FW-190 problems: It had 1x MG-FF 20mm in each wing with a 60 round drum (7 seconds of firing) Later models had them removed for better roll performance. Also a synchronized 151/20 mounted in each wing root with 250 rounds each. This resulted in a RoF of about 550 round per minute rather than 750 rounds. The inboard guns were mounted to the main fuselage spar next to the pilot. Firing them created intense vibrations in the cockpit which could result in decreased gunsight tracking while firing a sustained burst (bomber attacks). The spent rounds and belt needed to fall through a slot below the fuselage which could get jammed in high G turns while firing.
P-38 20mm problems: Used the US HS 20mm in the nose with 150 rounds. It had the same headspace problems. The 4x .50cal guns had 500 rounds. This was the most amount of rounds carried for a .50cal for a fighter allowing 40+ seconds of firing, great for long range missions.
Corsair 20mm problems: The outer wing (which folded up) flexed in high G maneuvers distorting the ammo feed boxes leading to a jam. If one cannon jammed it would yaw the aircraft throwing off the aim. The Navy rejected the 20mm for fighter duty but kept it for ground and naval attacks.
P-61 20mm problems: At first the 4x HS 20mm mounted under the fuselage created structural damage when fired and had to be reinforced. The HS 20mm gave the same problems and also suffered from freezing causing frequent jamming and the metal belts became brittle and snapped. This was fixed by installing gun heaters. The ventral mount picked up mud and debris when landing on unpaved strips on Pacific Islands fouling the guns and mechanisms. Firing sustained bursts created noxious gasses in the unpressurized cockpit and crew area.
Choices and Compromises:
20mm cannon and .50cal comparisons:
.50cal advantage vs 20mm: higher MV, greater accuracy, rate of fire and larger ammo load.
20mm advantage vs .50cal: Greater single hit damage.
.50cal API-T advantage: After penetration, the filler generated a brief hot incandescent flash exceeding thousands of degrees and kept burning which could melt fuels lines and ignite vapors.
Ground Support: 20mm
Air Combat: .50cal mainly because of increased ammo, especially for long range escorts.
20mm Fusing Problems: The early war 20mm HE round fuses were very sensitive and could explode against the skin of the aircraft and not penetrate before exploding. The German MG-FF round had this problem. It was later solved with a delay nose fuse.
The .50cal API-T round did not have a fuse. When the nose struck something solid the impact triggered the explosive. It exploded with a flash and then continued burning. The fuselage fuel tanks in the Bf-109 and FW-190 were behind and below the pilot. A round hitting the seat armor and not penetrating could end up sitting on top the fuel tank burning. This would ignite the fuel vapor.
Lethality: Some accounts state it took 3-5 hits from a 20mm HE rounds (fewer with the German 151/20) to down a single engine fighter in WWII while it may take 20-40 rounds of .50cal (fewer using API-T) and 250-500 .30cal rounds but could fire 8x .303 machine guns could fire 160 rounds per second so a close range 2 second burst would most likely be fatal. Of course shot placemen is important.
Solving the Jamming and Freezing Problems:
American: Had remote hydraulic or pneumatic chargers for the Wildcat and Hellcat. The 20mm armed Corsairs had a pneumatic charger. The P-61 had hydraulic chargers for the 20mm guns.
The Mustang and P-47 did not have chargers. If a gun jammed it was out until fixed on the ground. The reason was too much weight and not enough room in the wing.
The P-38 was more reliable because the guns did not suffer from wing flexing and was easier for ground crews to service. They also had better ammo feed systems which were short and straight and curved to feed through the top of the gun putting less pressure on the belt feed mechanism. However, being mounted in the nose meant more freezing conditions, especially for the 20mm cannon. Later electrical heat solved the problem. The .50cal guns had a cable charging system the pilot could pull in the cockpit. The 20mm had pneumatic charger.
The P-40 had a manual cable charging system on the floor of the cockpit for each wing gun. The pilot had to grab it and pull it to extract a shell. The nose mounted .50cal guns breech block extended into the cockpit.
The P-39 had manual cable charging system for all guns.
Japan: The Zero had a pneumatic charger for the wing 20mm cannons. The nose mounted 7.7mm guns breech block extended into the cockpit.
British: Spitfire and Hurricane had a pneumatic charging system for the 20mm cannons which were also suspectable to jamming from freeing. The Spitfire Mk I did not have a remote charging system for the .303 wing guns. During the BoB the main problem was not jamming but freezing at high altitude locking the firing pins in place. This was later solved by ducting warm air from the engine to the gun bays.
Russia: Most fighters had pneumatic chargers for wing guns and manual cable ones for nose guns.
Summary: It appears all nations saw the advantage of wing mounted 20mm cannon over machine guns. The issue was getting them to fit in the wing and work correctly. Also, the ammo load per gun was about 50% of machine guns severely limiting engagement times. Some favored the all MG layout to put as much lead per second at the target increasing the number of hits versus fewer hits but more damage. You needed cannons against bombers.
All wing guns seemed to have problems in one area or another except the .303 which was pretty reliable and losing one meant you still had seven. It appears the US could have fixed the problem with the HS 20mm gun but decided not to. The German 151/20 had the most devastating shell but it was heavy and had a lower muzzle velocity than the HS. All nations seemed to have problems with the spring operated ammo drums.
This can't all be correct so I'm open to corrections.
Excuse the long post, I plead guilty to being a gun nerd.
Wolfhag
paints up some WWII Soviet aircraft.
