Armor Penetration - 20mm vs. .50 cal.

[billswagger]

I certainly agree that the table shows that the trajectories of the two weapons are so similar over normal engagement distances so to make the difference almost irrelevant in normal combat. I also agree that the source of the table would be of considerable interest.
However your comment on the table seeming to assume that the 20mm had a markedly higher mv I find confusing. The results on the table are almost exactly what I would expect given the characteristics of the two weapons. Would you care to explain this comment you made?

Maybe the table is from a later time period.
The 20mm cannon was more reliable, had a better rate of fire, and more accurate post WW2. The .50 cal guns saw some improvements, but the scope of this discussion should remain in the context of WW2.
My only argument as for the reliability of the 20mm, is that it must've been faulty enough to prevent the US from wanting to use it on its early to mid war planes, although the British did continue to use it despite some defects and on going development of the gun. It must've been good enough for combat, but not reliable enough for US standards. Plus, Britain found itself defending against bombers where larger projectiles would've been preferred. Most mid war US planes were performing escort duties, so there wasn't a need for 20mm, if the .50 proved to be more accurate and reliable.
By the late war (1944-1945) the 20mm cannon was the preferred projectile because all of the problems since developing the cannon from pre-ww2, had been minimized. It had an equal rate of fire, and by late war standards, ground targets and their better armor were of increasing concern, which the .50 cal was not very effective.
I find it interesting, because they still used rockets and bombs in those situations, and the 20mm was spared for air to air uses, where .50 cal was as effective.

So maybe it did come down to weight and ammo load. But there are pilots who liked having the larger ammo loads of the .50 cal so they could be more liberal with their bursts. I'm sure there just as many reasons for preferring the 20mmm cannon in air to air combat.

 

[billswagger]

Hi Bill,
.

If you have data on the rate of stoppages of the war-time 12.7 mm Browning, bring it on - as far as I can tell, it was a gun like any other in that regard.

You seem to be better at diggin up information so you go ahead and take a crack at it, just try to be impartial, as i'm sure you are only trying to discuss facts.

 

[Magister]

Apparently, it wasn't until mid 1944 before the USN had worked out the "temperamental characteristics" of the 20mm gun and they had an "awful lot of headaches" related to employment of the 20mm.

Joint Fighter Conference, p, 157.

I don't have my fingers on the page(s) but I do recall there being statements lauding the reliability of the .50.

Of interest, the following was also said:

"We have developed some wonderful strafing techniques. They are sinking destroyers and they have stopped cruisers dead in the water with .50 cal. gun fire. It's a tremendous weapon. When we get these 20mms out there, it is going to be even tougher. They can put a pretty good sized ship out of commission with them."

Joint Fighter Conference, p, 159.

 

[Graeme]

What's the source of the table, by the way?

G'day HoHun. It's from a RAAF gunnery manual dated September 1943 and a chapter devoted to harmonization. It was my layman belief that if a projective is affected less by gravity and therefore required minimal harmonization correction it would therefore be the basis of a better weapon (compared to the 0.50 MG).

 

[usafmd]

Does any of this take into account the explosive 20 mm round's effect?

 

[Glider]

Maybe the table is from a later time period.
The 20mm cannon was more reliable, had a better rate of fire, and more accurate post WW2. The .50 cal guns saw some improvements, but the scope of this discussion should remain in the context of WW2.

I totally agree.

My only argument as for the reliability of the 20mm, is that it must've been faulty enough to prevent the US from wanting to use it on its early to mid war planes, although the British did continue to use it despite some defects and on going development of the gun. It must've been good enough for combat, but not reliable enough for US standards.

US built 20mm HS404 based guns and their ammunition were far less reliable than UK built weapons due to changes that were made by the UK to the original French design. These changes were no made to the US weapons. No one is saying that the UK 20mm was perfectly reliable but it was probably no worse than any other mass produced weapon of a similar type in this period. The thread listed earlier gave examples of the firing tests held in the USA. As for the ammunition the best example I can give is on Malta. They had a huge number of failures and when it was looked into the fault lay with a batch of US ammunition. As a result and despite the desperate shortage of all types of ammunition Malta destroyed all US manufactured 20mm ammo. The problem then went away.

Plus, Britain found itself defending against bombers where larger projectiles would've been preferred. Most mid war US planes were performing escort duties, so there wasn't a need for 20mm, if the .50 proved to be more accurate and reliable.

Britain decided before the war that the LMG wasn't effective enough and decided to go for the 20mm. The 8 x LMG was almost a fall back position which by the standards of 1938 was in itself was more than respectable. These decisions for the UK and USN (amongst others) were taken before the war, the UK going for the 20mm and the US the 0.50 M2. Decisions taken during the war tended to be the result of lessons learnt and/or the aircraft available to the opposing forces. The fact that the UK faced bombers was no doubt a factor but not critical, after all the USN also faced bombers.
For the US forces the 0.5 was more than sufficient for what they wanted it to do and there was no reason for change. Had the US been facing large numbers of B17 or B29 style bombers then its my belief that they would have made changes, but they didn't need to, so why change?

By the late war (1944-1945) the 20mm cannon was the preferred projectile because all of the problems since developing the cannon from pre-ww2, had been minimized. It had an equal rate of fire, and by late war standards, ground targets and their better armor were of increasing concern, which the .50 cal was not very effective.
I find it interesting, because they still used rockets and bombs in those situations, and the 20mm was spared for air to air uses, where .50 cal was as effective.

The armour on ground targets is I think an exagerated point, the 20mm did have better penetration but not enough to damage the majority of targets. It might make a difference if you targeted a light armoured car and got lucky, but not really. If you are attacking a truck then a bullet might knock a hole in it and if it hit a none vital part, do no more, however a 20mm is almost certain to wreck it.

So maybe it did come down to weight and ammo load. But there are pilots who liked having the larger ammo loads of the .50 cal so they could be more liberal with their bursts. I'm sure there just as many reasons for preferring the 20mmm cannon in air to air combat.

If I can break this into two parts.
Endurance of the ammunition load
I agree this is a valid point often ignored. If you are on a long escort mission or at sea where you have to stop the attacking aircraft then how long you ammo lasts is vital. If you are defending with other airfields to divert to then the amount of ammunition you can fire in a burst is probably more important.
I use the example of the Whirlwind. With 4 x 20mm in the nose, for 1940 its firepower was unmatched by any day fighter. But it only had 6 seconds of ammo, say three bursts. If you are attacking a bomber formation when three accurate bursts, each of which may well destroy or seriously damage a bomber, then that is probably fine. On the other hand, would you like to go on a four/five hour mission, hundreds of miles behind the lines with only three bursts of ammo?
Weight of the Ammo / Weapons
This is basically efficiency. In numbers the 0.5 may well be less efficient, but it was more than adaquate and efficient enough for the job in hand.
What is often forgotten is that the 0.5 M2 was a 1930 (ish) design based on a 1919 design. The 20mm Hispano II was a 1940 design (once they made it reliable) based on a 1937 French design. It would be a minor miracle if the 0.5 M2 was as efficient as later 20mm.

 

[Magister]

The fact that the UK faced bombers was no doubt a factor but not critical

Uh oh, here it comes.

 

[HoHun]

Hi Billswagger,

>You seem to be better at diggin up information so you go ahead and take a crack at it, just try to be impartial, as i'm sure you are only trying to discuss facts.

I can assure you that I'm keeping my eyes peeled for information on the reliability of the 12.7 mm Browning

However, since anecdotal accounts show that it was not literally "perfect" either, I think that at least some of the responsibility of finding quantified data would rest with the defenders of the Browning.

Way to go on the "cannon calibre" smiley, by the way - good sense of humour there!

Regards,

Henning (HoHun)

 

[HoHun]

Hi Graeme,

>G'day HoHun. It's from a RAAF gunnery manual dated September 1943 and a chapter devoted to harmonization.

Thanks a lot! Is that a find from the Australian Archives as well? Obviously, I'd love to get my hands on that!

>It was my layman belief that if a projective is affected less by gravity and therefore required minimal harmonization correction it would therefore be the basis of a better weapon (compared to the 0.50 MG).

Quite right, but the thing that surprised me about the data in the table you posted is that the drop is greater for the 12.7 mm Browning than for the 20 mm Hispano, which would indicate that the muzzle velocity of the Hispano was assumed to be greater than that of the Browning for that table. From the data I have seen, the difference is not big but in favour of the Browning, so I'd have expected the opposite relationship. I'm not sure what to make of this.

Regards,

Henning (HoHun)

 

[HoHun]

Hi Usafmd,

>Does any of this take into account the explosive 20 mm round's effect?

As far as my posts are concerned, the explosive/incendiary effect of all rounds is taken into effect by calculating the total muzzle energy (kinetic + chemical, based on the mass of the chemical content multiplied with the specific energy of TNT).

(The increase in the vulnerable area of the target to high-explosive 20 mm shells that is pointed out by German trials is not taken into account and would further balance the scale in favour of cannon.)

You can find a more detailed comparison of a variety of aircraft guns here:

http://www.ww2aircraft.net/forum/te...mparison-machine-guns-light-cannon-17521.html

The basic method is described here (two different methods actually):

WORLD WAR 2 FIGHTER GUN EFFECTIVENESS

Regards,

Henning (HoHun)

 

[Magister]

This may have already been covered earlier in the thread so I apologize if ai am preaching a repeated sermon.

Muzzle velocity is only one component affecting bullet drop. Ballistic coefficient is another. Very generally speaking, bullet drop is caused by gravity. Drag is what slows the bullet down. Time of flight is greater if there is more drag and thus, gravity has a longer period of time in which to act on the projectile. Gravity is a constant. For a given bullet fired at a known muzzle velocity, the ballistic coefficient of the bullet determines its trajectory because ballistic coefficient is the largest factor governing the amount of drag.

Thus, you can have a 20mm projectile starting out at a higher muzzle velocity than a .50 cal projectile and have the .50 arrive on target sooner and with less drop than the 20mm.

Without the BC, you can't measure bullet drop at range. Charts are nice but limited by the preparer's knowledge of relevant factors and the availability of data.

 

[tomo pauk]

Magister, the theory you write lacks a key factor: the projectile weight. The heavier the projectile, the better it retains velocity within atmosphere.

 

[Glider]

Quite right, but the thing that surprised me about the data in the table you posted is that the drop is greater for the 12.7 mm Browning than for the 20 mm Hispano, which would indicate that the muzzle velocity of the Hispano was assumed to be greater than that of the Browning for that table. From the data I have seen, the difference is not big but in favour of the Browning, so I'd have expected the opposite relationship. I'm not sure what to make of this.

Regards,

Henning (HoHun)

Henning
You seem to be working on the assumption that if a projectile has a lower MV then it has a bigger drop. This is not the case, certainly the MV is a factor but there are a number of other factors, probably the most important being the Sectional Density. This is the relationship between the mass of the projectile and the Frontal Surface Area of the projectile.

As you would expect there are others such as the aerodynamics but all else being equal, a larger projectile will have an advantage over a smaller one. Hence my previous statement that as the MV is similar between the 12.7 and the 20mm, the results are what I would expect.

 

[Magister]

The weight of a projectile is a component of of sectional density (SD) which is a component of ballistic coefficient (BC). I just skipped to BC.

Ballistic coefficient: BC=SD/i (i is the form factor [shape] of a bullet).

I also did not mention that temperature, humidity, barometric pressure and altitude are also factors.

 

[Glider]

The weight of a projectile is a component of of sectional density which is a component of BC.

Ballistic coefficient: BC=SD/i (i is the form factor [shape] of a bullet).

I also did not mention that temperature, humidity, barometric pressure and altitude are also factors.

Totally agree

 

[Magister]

There are few things as dangerous as someone who isn't equipped to understand a subject of inquiry who decides to play around with computer software.

 

[HoHun]

Hi Timshatz,

>It makes you wonder, if the Japanese could scale up the M2 to a 20MM and get good results, why the US Military couldn't do it as well. My understanding is they were having trouble with the US "homegrown" 20MM all through the war.

I looked it up in Tony's "Rapid Fire" now:

"The Ho-5 was yet another version of the Browning scaled up to take a 20 x 94 cartridge which was in effect a lengthened MG 151/20 case (or a shortened Hispano!). The gun weighed only 37-45kg (depending on type) and achieved 850 rpm which made it, on paper at least, one of the best of the wartime cannon."

I have included the Ho-5 in my firepower comparison here:

http://www.ww2aircraft.net/forum/te...mparison-machine-guns-light-cannon-17521.html

The standard comparison for equal total ammunition energy:

4x 20mm Ho-5 - 370 rpg, 26 s duration - 539 kg - 2,8 MW firepower
8x ,50 Browning M2 - 425 rpg, 33 s duration - 694 kg - 2,3 MW firepower

Firepower: Ho-5 better by 22 %
Weight: Ho-5 down by 342 lbs

As the Ho-5 replaced the 12.7 mm Ho-103, itself a Browning copy, as nose gun in the Ki-61, it seems to have been a fairly compact weapon too.

Regards,

Henning (HoHun)

 

[Graeme]

Is that a find from the Australian Archives as well?

Hi HoHun.

I guess you could call it that! The manual belonged to my father when he was training to be an air gunner in 1944. It was an introduction to air gunnery discussing various aspects of fixed and turret guns and he was training to be a gunner in a Hudson bomber but the War direction changed. It's also sprinkled here and there with the odd exercise for the ardent student...

 

[Clay_Allison]

Hohun, did you have any thoughts about my proposed fix for US 20mm cannon (actually cartridge) design?

 

[HoHun]

Hi Clay,

>Hohun, did you have any thoughts about my proposed fix for US 20mm cannon (actually cartridge) design?

I believe the Japanese actually adopted a different cartridge (instead of modifying the original one) for their Ho-5, but your suggestion certainly makes sense as the German MG 151 was converted in a similar way to the 20 mm version MG 151/20, allowing the calibre to be changed by simply swapping the barrel. I don't see any reason why your suggestion shouldn't have worked as intented!

Regards,

Henning (HoHun)