50 cal (high rate of fire) vs 20mm cannon (hitting power)

[A.G. Williams]

T

The Hispano mechanism should have been suitable for synchronisation.

It wasn't though, because it was designed to fire from an open bolt. The only one of the Hispano 404-based family which could be synchronised was the US M24, as that had electrical priming (like the MG 151/20E).

 

[Shortround6]

The original French gun (and British copy, the Mk.I) fired at about 700 rounds/minute, but after various trials in 1939 it was found that reliability and breakages were greatly improved if the rate of fire was kept down to about 600 rounds/minute -- so this was specified in the Mk.II gun.

This was a choice some countries made for several different guns. Slower rate of fire for increased reliability and/or durability. Sometimes increased rate of fire was chosen instead. It rather depended on the country's "standard" for reliability and/or durability. Sometimes modifications in the form of redesigned-stronger parts or improved materials (alloys/heat treating) allowed for later increases in rates of fire in later guns.
Since the acceptable number of jams and broken parts per 1000 (or 10,000) rounds fired is seldom known in popular books it does make comparison difficult.
This is something that delayed the introduction of the 1050-1200rpm .50 cal Browning machinegun in US service. Many of the early development models could not reach (some by a rather large margin) the desired reliability standard.

 

[Koopernic]

A combination of lighter reciprocating parts with stronger (or extra) recoil springs is the universal way to increase RoF. With gas operation (including the Hispano) a lot can be done with the location and size of the gas port: the closer it is to the chamber, the faster the bolt will unlock.

if the lighter less powerful MG151/20 round had of been fired from a Hispano style mechanism what weight reduction and what ROF increase might have been possible? is there a rule, like an inverse of say momentum decrease?

 

[jmcalli2]

A bit of nit-picking: the standard .50 cal ball didn't have a lead core, it had a steel one in a thin lead sleeve. From 1944 onwards, the favoured bullet type for fighter aircraft was the M8 API; this had a hardened steel armour-piercing core with a quantity of incendiary material in the jacket tip. It was common to make every fourth or fifth round an M20 API-T (tracer). For bomber defence, the .50 often used the M21 "Headlight" tracer; it had been discovered that attacking Luftwaffe pilots could be distracted by seeing the tracers coming towards them, so the M21 was designed to have a big, bright tracer which could easily be seen from the front.

The standard RAF 20mm Hispano belt make-up from mid-war onwards was two HEI followed by two SAPI (semi-armour-piercing-incendiary). The HEI had a fuze with a slight delay, to ensure that it exploded inside the target rather than on the surface. The SAPI shells used the same shell body as the HEI, but were filled with a large quantity of incendiary material and were given a penetrating steel cap instead of a fuze; the incendiary was ignited by the shock of impact. They both worked pretty well, once the early problems with over-sensitive fuzes were dealt with (the Luftwaffe had the same problem with their 20mm HE).

Just remembered I had these from the PT boat exhibit at the USS Massachusetts. I apologize for the low res pics; I lost many from this trip and had to download low res version I had posted.

 

[A.G. Williams]

if the lighter less powerful MG151/20 round had of been fired from a Hispano style mechanism what weight reduction and what ROF increase might have been possible? is there a rule, like an inverse of say momentum decrease?

It's impossible to say - the devil is in the detail.

Anyway, weight comparisons between the Hispano and MG 151/20 are complicated. A couple of extracts from my new book (next year)

On the MG 151/20: "The gun weighed 42 kg including not just the integral belt feed but also the electrical charging device and electric trigger, without which the gun weighed 36-37 kg. It had a cyclic rate of 700 rpm."

On the US Hispano: "The detailed weight breakdown for the AN-M2 is interesting, as the gun's construction was unusually modular. It weighs 46.3 kg as a bare gun, but the muzzle brake (not used with belt feed) weighs 2.1 kg, the front mounting adapter between 3.4 and 6.4 kg depending on the model, the electric trigger and sear mechanism 0.8 kg, the hydraulic charger 1.2 kg, and the ammunition feed 10 kg (60-round drum) or 8.4 kg (belt feed drive), giving a total weight of 60-67 kg depending on the feed mechanism and the mounting adapter. The mounting or support cradle would be additional to this."

 

[tomo pauk]

A couple of extracts from my new book (next year)

I've heard that 2021. will be a good year

 

[Koopernic]

It's impossible to say - the devil is in the detail.

Anyway, weight comparisons between the Hispano and MG 151/20 are complicated. A couple of extracts from my new book (next year)

On the MG 151/20: "The gun weighed 42 kg including not just the integral belt feed but also the electrical charging device and electric trigger, without which the gun weighed 36-37 kg. It had a cyclic rate of 700 rpm."

On the US Hispano: "The detailed weight breakdown for the AN-M2 is interesting, as the gun's construction was unusually modular. It weighs 46.3 kg as a bare gun, but the muzzle brake (not used with belt feed) weighs 2.1 kg, the front mounting adapter between 3.4 and 6.4 kg depending on the model, the electric trigger and sear mechanism 0.8 kg, the hydraulic charger 1.2 kg, and the ammunition feed 10 kg (60-round drum) or 8.4 kg (belt feed drive), giving a total weight of 60-67 kg depending on the feed mechanism and the mounting adapter. The mounting or support cradle would be additional to this."

How long did it take to "charge" the gun if it jammed due to a failure to extract, fire or load? WW2 fighter pilot anecdotes are full of stories of quarries that escaped due to a jamed guns.

Also could the jam be detected and the charge initiated automatically? For instance if the firing trigger was pressed and a timer detects a failure to cycle within a certain amount of time could the charging be initiated without pressing a separate button?

The hydraulic charging for the US Hispanic suggests that a solenoid valve opens a hydraulic line to charge the gun (or all guns simultaneously) when a button is pressed.
Electrical system possibly simpler but possibly slower. The Germans faced copper shortages so wonder if they used aluminium coils.

 

[A.G. Williams]

How long did it take to "charge" the gun if it jammed due to a failure to extract, fire or load? WW2 fighter pilot anecdotes are full of stories of quarries that escaped due to a jamed guns.

Also could the jam be detected and the charge initiated automatically? For instance if the firing trigger was pressed and a timer detects a failure to cycle within a certain amount of time could the charging be initiated without pressing a separate button?

The hydraulic charging for the US Hispanic suggests that a solenoid valve opens a hydraulic line to charge the gun (or all guns simultaneously) when a button is pressed.
Electrical system possibly simpler but possibly slower. The Germans faced copper shortages so wonder if they used aluminium coils.

I don't have any specific information about charging time, but I expect it would be fast. It was obviously not all that important, however, as omitting the charger was one of the weight-saving measures adopted for the Mk V. The guns were loaded and cocked on the ground.

In those pre-electronic days I don't believe there would have been any automatic detection or rectification of firing problems.

 

[Brooke]

If you were a fighter pilot in WWII, would you rather have the high rate of fire of the 50 cal, or the hitting power of the 20mm? I personally feel the 50 cal was plenty hard hitting enough to take out ANY aircraft, and its high rate of fire made it even more effective...the slow rate of fire for the 20mm meant you had to be a much better marksman...

In the late 1950s I was in the Air Scouts and we met a Moffett Field Naval Air Station in Mountain View, California. At one of the meetings at the side of an F-8 Crusader jet we were told how it had shot itself. For gunnery practice a barge was used as a target while being towed, i.e. a moving target. The F-8 would dive at the target while firing the forward facing .50 cal guns, continue the dive, level out and fly over the target. The problem was the plane got ahead of the bullets which hit the F-8. This was the motivation to switch to 20 mm. It was not related to rate of fire but rather to get faster bullets.

While studying W.W.II torpedoes it became oblivious that when the probability of a hit goes down as the travel time of the weapon goes up. I maintain that dumb weapons fail to work when the travel time is too long. This applies to torpedoes, big guns, bombs, &Etc.
See: Torpedoes

 

[swampyankee]

In the late 1950s I was in the Air Scouts and we met a Moffett Field Naval Air Station in Mountain View, California. At one of the meetings at the side of an F-8 Crusader jet we were told how it had shot itself. For gunnery practice a barge was used as a target while being towed, i.e. a moving target. The F-8 would dive at the target while firing the forward facing .50 cal guns, continue the dive, level out and fly over the target. The problem was the plane got ahead of the bullets which hit the F-8. This was the motivation to switch to 20 mm. It was not related to rate of fire but rather to get faster bullets.

While studying W.W.II torpedoes it became oblivious that when the probability of a hit goes down as the travel time of the weapon goes up. I maintain that dumb weapons fail to work when the travel time is too long. This applies to torpedoes, big guns, bombs, &Etc.
See: Torpedoes

From all sources I've seen, the Crusader was never equipped with 0.5 in machine guns; the USN stopped using those as main armament on its aircraft when all its jets had straight wings. There was at least one aircraft that shot itself, which was an F11F Tiger. If I remember, its guns were fired while it was in a shallow climb, then the pilot transitioned to a shallow dive. Alas, at least one or two of the 20 mm shells trajectory intersected with the aircraft's flight path. The FJ-1 Fury, FH Phantom, and the F6U Turd Pirate were the last USN fighters armed with 0.5 in guns; the F2H Banshee, F9F Panther/Cougar, and FJ-2 Fury (which had approximately nothing in common with the FJ-1 Fury) all had 20 mm guns.

 

[gomwolf]

US Ballistic Research Laboratories tested some WW2 fire arms in 1947. Title of the reprot is "Airplane Vulnerability and Overall Armament Effectiveness".
They tested 50cal, 60cal(US copied MG151/15), 20mm Hispano, 3cm MK108, 37mm M4.
I am quite sure you will find your own conclusion after read this report.

Personally I believe bigger HE shell is more effective to aircraft. Cuz it can destroy aircraft's structure itself even if shell didn't hit the vital parts.

 

[A.G. Williams]

In the late 1950s I was in the Air Scouts and we met a Moffett Field Naval Air Station in Mountain View, California. At one of the meetings at the side of an F-8 Crusader jet we were told how it had shot itself. For gunnery practice a barge was used as a target while being towed, i.e. a moving target. The F-8 would dive at the target while firing the forward facing .50 cal guns, continue the dive, level out and fly over the target. The problem was the plane got ahead of the bullets which hit the F-8. This was the motivation to switch to 20 mm. It was not related to rate of fire but rather to get faster bullets.

Aircraft did occasionally shoot themselves down, either by overtaking their projectiles and flying into them, or (more often, I think) by ricochets bouncing back into their flight path after a ground strafing run. Some TP projectiles were made frangible specifically to remove the latter risk - they shattered on impact with the ground.

However, that had nothing to do with the choice of armament calibre. At the end of WW2 the USAAF's preferred ammunition for future fighter armament was the .60 cal derived from a very high-velocity experimental anti-tank rifle round. Prototypes of both the 1950s revolver cannon (which later became the M39) and the M61 Vulcan rotary were made in this calibre, but dropped in favour of necking the case up to 20 mm, as obtaining the blast effect of a cannon shell was recognised as being worth losing some velocity to obtain.

 

[A.G. Williams]

While studying W.W.II torpedoes it became oblivious that when the probability of a hit goes down as the travel time of the weapon goes up. I maintain that dumb weapons fail to work when the travel time is too long. This applies to torpedoes, big guns, bombs, &Etc.
See: Torpedoes

That is clearly correct, as the shorter the flight time the easier it is to hit targets, and it did for a while drive a USAAF obsession with maximising the muzzle velocity of aircraft guns. However, it was eventually realised that explosive cannon shells were more important. No other air force - and not even the USN - questioned the superiority of cannon fire over MGs.

 

[Koopernic]

I don't have any specific information about charging time, but I expect it would be fast. It was obviously not all that important, however, as omitting the charger was one of the weight-saving measures adopted for the Mk V. The guns were loaded and cocked on the ground.

In those pre-electronic days I don't believe there would have been any automatic detection or rectification of firing problems.

Googling "automatic remote cocking mechanism" brings little up re WW2 weapons though Denel seem to have something for 50 caliber guns that are remotely operated from armoured vehicles. I imagine it could be done electromechanicaly fairly easily. For instance if the gun trigger is pressed it also closes a switch to rotate a small electric motor. This motor turns a cam through a clock work mechanism and a clutch. If the cam manages to rotate say 180 degrees it will close a switch that cocks and charges the canon and disposes of the dud or jamed round. If the canon however cycles properly it will momentarily open the clutch through a lever as the bolt or barrel moves.

That what seems a simple automatic jam clearing mechanisms like this didn't exist suggests a different thought process more along the lines of "we will make a weapon so reliable it won't need recocking in flight". Uktimatly that was the revolver canon.

PS, who came up with the idea of resurrecting the Gatling mechanism?

 

[jmcalli2]

US Ballistic Research Laboratories tested some WW2 fire arms in 1947. Title of the reprot is "Airplane Vulnerability and Overall Armament Effectiveness".
They tested 50cal, 60cal(US copied MG151/15), 20mm Hispano, 3cm MK108, 37mm M4.
I am quite sure you will find your own conclusion after read this report.

Personally I believe bigger HE shell is more effective to aircraft. Cuz it can destroy aircraft's structure itself even if shell didn't hit the vital parts.

I'm leaning towards rate of fire as a more important factor for one reason: how far is it between shells?
Say you have a gun firing 1,200 rnds/minute; that's 20 rounds per second (1,200 rounds per minute divided by 60 seconds per minute equals 20 rounds per second).
If the muzzle velocity is 2,400 ft/sec, then there are 120 feet between rounds (2,400 feet per second divided by 20 rounds per second equals 120 feet between rounds).

A M61 Vulcan fires ~3,000 rnds/min at ~3,000 ft/sec; there are 60 feet between rounds.

A M2 .50cal fires ~800 rnds/min at 2,700 ft/sec; 202 feet between rounds!

A M4 37mm fired 150 rnds/min at 2,000 ft/sec; 800 feet between rounds!

Makes me wonder how these guys ever put more than one round on any target!

 

[tomo pauk]

I'm leaning towards rate of fire as a more important factor for one reason: how far is it between shells?
Say you have a gun firing 1,200 rnds/minute; that's 20 rounds per second (1,200 rounds per minute divided by 60 seconds per minute equals 20 rounds per second).
If the muzzle velocity is 2,400 ft/sec, then there are 120 feet between rounds (2,400 feet per second divided by 20 rounds per second equals 120 feet between rounds).

A M61 Vulcan fires ~3,000 rnds/min at ~3,000 ft/sec; there are 60 feet between rounds.

A M2 .50cal fires ~800 rnds/min at 2,700 ft/sec; 202 feet between rounds!

A M4 37mm fired 150 rnds/min at 2,000 ft/sec; 800 feet between rounds!

Makes me wonder how these guys ever put more than one round on any target!

If the pilot is a good marksman, and/or has excellent sight (late ww2 ones were far better than the ones from 1920s/30s), and/or he is a very good pilot thus can fly very close to the target, he will land burst on it's target. Shells/bullets from the burst will stand a good chance to hit in close proximity.
If nothing of the above is present, his bursts will never land on target, even if he has a 10000 rd/min gun.

(the chaps from Zagreb-Borongaj barracks managed to sever the line attaching to the target drogue and the aircraft, carried the parts of the line as a trophy back to the barracks; gun in question was the powerful 30mm of Czech origin; our crews - from Ljubljana Polje barracks, where I've also served - never managed it)

 

[BiffF15]

I'm leaning towards rate of fire as a more important factor for one reason: how far is it between shells?
Say you have a gun firing 1,200 rnds/minute; that's 20 rounds per second (1,200 rounds per minute divided by 60 seconds per minute equals 20 rounds per second).
If the muzzle velocity is 2,400 ft/sec, then there are 120 feet between rounds (2,400 feet per second divided by 20 rounds per second equals 120 feet between rounds).

A M61 Vulcan fires ~3,000 rnds/min at ~3,000 ft/sec; there are 60 feet between rounds.

A M2 .50cal fires ~800 rnds/min at 2,700 ft/sec; 202 feet between rounds!

A M4 37mm fired 150 rnds/min at 2,000 ft/sec; 800 feet between rounds!

Makes me wonder how these guys ever put more than one round on any target!

jmcalli2,

The M61A1 Vulcan cannon (designed by General Electric - "We bring good things to life") as installed in F15, F16, & F18 fires at a cockpit selectable rate of either 3 or 6k/min. In the Eagle we kept it on 6k. Total rounds count in the light grey Eagle is 940. The lesser planes carried less...

Cheers,
Biff

 

[Shortround6]

I'm leaning towards rate of fire as a more important factor for one reason: how far is it between shells?
Say you have a gun firing 1,200 rnds/minute; that's 20 rounds per second (1,200 rounds per minute divided by 60 seconds per minute equals 20 rounds per second).
If the muzzle velocity is 2,400 ft/sec, then there are 120 feet between rounds (2,400 feet per second divided by 20 rounds per second equals 120 feet between rounds).

A M61 Vulcan fires ~3,000 rnds/min at ~3,000 ft/sec; there are 60 feet between rounds.

A M2 .50cal fires ~800 rnds/min at 2,700 ft/sec; 202 feet between rounds!

A M4 37mm fired 150 rnds/min at 2,000 ft/sec; 800 feet between rounds!

Makes me wonder how these guys ever put more than one round on any target!

I think you are looking at this from the wrong direction.

If a plane doing 300mph (440fps) flies in front of a gun firing at 20 rps and does so in a 90 degree path to the gun then the gun will put a bullet into the airplane every 22 ft of it's length.

This is regardless of the speed of the bullet or the distance to the target (for practical purposes). A 600rpm (10 rps) gun gets a hit every 44 ft.

as the angle changes the bullet holes will get closer together. For instance (If i have done the math right) a plane doing 300mph (440fps) and flying in front of our fixed gun will take a bullet hole evey 11 ft from the 20rps gun and one every 22 ft from the 10rps gun. The plane takes twice as long to get through the 'danger space" due to the long distance flown. Obicouls it will take a very lucky hit to bring down a plane even with multiple guns. However the gun/s are not fixed in space and they are supposed to be tracking the target (changing their angle between shots)

Both factors are important. High velocity comes into play, not to decrease the distance between bullets but to reduce the time of flight from when the bullet leaves the barrel until it gets to the target (or the area the target is in). The pilot (or gunner) will try to aim/move his gun/s so they are pointed ahead of where the target is at the moment of firing and try to get the bullets into the part of the sky where hi thinks the target plane will be in 1/4 to 1/2 a second (our 300mph plane will move 110-220 ft in that time), The less distance the target moves before the bullets get there the greater the chances for hits. trying to estimate exact course, speed and altitude change (plane goes up or down 10 ft in that fraction of a second the bullets take to get there?)

 

[Admiral Beez]

A couple of the heavier British cannon (Vickers 'S', Mollins 'M') are sometimes referred to as 2pdr and 6pdr respectively -- but I think the official designations stuck with metric.

But why define cannons by mass? I know that's naval tradition.

 

[A.G. Williams]

But why define cannons by mass? I know that's naval tradition.

Not just naval. During WW2 the British Army had the 2 pdr and 6 pdr tank/anti-tank guns as mentioned, plus the 17 pdr T/AT and the 25 pdr and 60 pdr artillery pieces.

However, it is true that starting with the introduction of the Hispano, the RAF favoured metric designations in all new aircraft guns, with the exception of the 6 pdr Molins.

I must admit I don't understand the British nomenclature at that time. The army had three light field artillery pieces in service in WW2: the 25 pdr Field Gun (87mm calibre ), 95 mm Infantry Howitzer (94mm), and 3.7 inch Mountain Howitzer (94mm).