Here's a table that the British produced to show the (in)effectiveness of their tank guns against the main German medium tanks:
Item No.3 is the Panzer III Ausf. H and item No.4 is the Panzer III Ausf. J
However, I have not seen an equivalent table showing how effective or not the 5cm was against British tanks. Using Liddell-Hart's figures against the main British-made tanks, and using the standard German APCBC-HE ammunition, I reckon that the 5cm KwK 39 in the Panzer III was roughly this effective:
Front (75mm) Nil
Side (65mm) Nil
Front (60mm) <200x
Side (60mm) <200x
Front (50mm) <500x
Nose Plate (32mm) <1500x
(These figures at 30 deg.)
If Liddell-Hart's figures are correct, it shows that the 5cm KwK 39 was only really effective against a limited portion of the front of the Crusader. The following picture shows the portions of the front of the Crusader that fell below the nominal 50mm basis, highlighted in a lovely shade of lilac:
These areas are the nose plate (lowest), the parts of the driver's front plate not covered by the cast visor and revolver port, and the nearside upper front plate. However, this latter plate would have been additionally covered by the hull MG turret (whether occupied or not) up until Spring 1942. Nonetheless, these are the areas that German gunners would have had to have aimed for if they wanted to disable the Crusader from a distance.
Now, one thing that is missing from the British table is that although the Panzer III Ausf. H had the front of its hull built up to 60-62mm with face hardened plates, my understanding (which may or may not be correct) is that its front turret plates remained at 30mm thick, and its gun mantlet remained at 35mm thick. I don't know how the armour in this area worked, but if these were the actual thicknesses, this implies that the turret of the Ausf. H was vulnerable to the 2 pounder at 1000x to 1500x.
I have not seen any documents generated by the RAC or the GHQ(AFV) in Cairo that mention the turret front as a vulnerable area, which is curious, and it would be interesting to know if the RA had anything to say about it. However, if this area was indeed in the 30 to 35mm range of thickness, it would mean that the Panzer III Ausf. H was at least as vulnerable at range as the Crusader. And if this was considered to be too small an area to deliberately target at long range by the British, why would the Germans attempt to target the small vulnerable plate area of the Crusader at long range?
Is it the case then that the real difference was not in the relative effectiveness of the tanks, but in the fact that the Germans effectively coordinated their tank and anti-tank units, whereas the British didn't?
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"The demonstration, as a demonstration, was a failure. The sunshield would not fit the tank. Altogether it was rather typically Middle Easty."
- 7th Armoured Brigade War Diary, 30th August 1941