German guns vs. Matilda II: help needed

[delcyros]

The Germans were hoisted by their own petard of their penetration definition. The US (and/or British) tested the early 88mm AP penetration and it didn't come away so badly. But the US definition of a penetration was that more than 50% of the shell pass completely through the armor, regardless of the condition of the explosive charge.

The 50% feature -as I understand- is the US naval ballistic limit. The US Army Ballistic limit -similar to the britisch penetration definition- required only a crack through the plate from one side to the other. The projectile didn't need to pass.

 

[Mobius]

I haven't heard that British definition before. But my notes say that the definition evolved over time. 1. The pre war criteria used for the 2-pdr seems to be to be that 80% chance of 'success'. (In the case of a CV Limit -- Critical Velocity -- the definition for projectile "success" is that at least 20% of the projectile must end up past the rear surface of the plate as a "free missile".)

2. Later for the 6-pdr they did some proof testing vs. armor of specific thickness. (For the projectile to pass this proof of shot test, 66% of the shot (66% by weight) had to pass completely through the plate at the specified impact velocity.)
Later:
3. Per March 1944 British Tank School Definition of CV. The success was defined as a 50% chance of CV Limit success (as opposed to the 80% in the 2-pdr definition.)

[edit]
It occurs to me that we don't need to know exactly what the British criteria is if we can at least compare the same gun penetration to a set of data we do know the criteria.

About the same time frame the British must have been testing the 75mm guns and ammo they received from America.
If we assume the British tested the 88mm using the same criteria as they did the American 75mm we can determine a ratio between the British test results and the American test results. We use that same ratio to determine the 88mm penetration in US criteria.

 

[delcyros]

Mobius,
proof testing is not penetration limit.
The requirements for service proof (acceptance) and penetration graphs were different.
You need to have a really POOR projectile to fail a proof trial.

 

[Mobius]

Not much luck trying to work backwards by comparing penetration sets to get an idea of the comparative criterias between the American and British. There are numerous typographical errors and mistakes in some War Office documents giving a wide range of values. Comparison of the US M3 75mm finds the British values are on average about 9% higher. The British data on the US 76mm is about 3% higher.

The British finds the M2 is about 6% higher than the US does if the correct 1850 ft/s MV is used. Somewhere along the line 1929 ft/s was used as a MV by some secondary sources.

 

[sgtleehead]

Difficult subject to determine because of a huge number of variables, including shell variables, more so perhaps than the gun itself. The British had used different criteria for gun penetration testing between 1939 to 1942. British figures generally tend to come out slightly higher. The British generally regarded German armour of poorer quality - hard but brittle with a tendency to crack because of its composition. This could account for the variables, ie a tendency to slightly increase the expected degree of penetration to allow for a suspected inferiority in the armour. This 'predicting' or modifying did sometimes happen in the absence of proper or consistent figures - see my next comments.

The shell though is a massive issue -quality control, powder strength and quality, metal quality etc. American rounds tended to always be slightly worse than expected. The 17 pdr was another classic example. The actual variance in penetration was off the scale - that's providing the round hit. APDS accuracy was so bad a lot of units wouldn't risk using it . But when it hit! - The figures were sometimes astonishing - but also so varied as to be not reliably document-able.