7,5 cm Kwk/StuK/Pak 40 Firing Table Data

[critical mass]

Yes quiete so. Pzgr.39 can be found in soft soils unexploded. Whenever it is recognized, it is not to be touched and blown up by a hollow charge without contacting the projectile.

[Yoozername]

On a shell collector website, Pzgr 39 are the third most found UXO according to EOD tech there.

In physics terms, it might be called 'jerk' or the rate of change of acceleration with time. In any case, I could see the pzgr 39 going through both sides of a halftrack. Anyone in the way would be splattered. Same as a gunshield, men are in a huddle behind it. A HE round on delay might be better.

A thick bunker might set it off.

[critical mass]

It also means that spaced armor needs to be thicker than 15>20mm RHA to be effective vs 75mm Pzgr.39.

[Peasant]

I thought that comparing the performance of 7.5cm PzGr.39 against german and british RHA might give some insight on respective nations penetration criteria. Seems like british plate behaves poorly at normal incidence when the T/D ratio is close to 1. I've checked the '45 chart as well and the difference between them seems minimal.

[critical mass]

Most likely, I don´t see much of a difference. The british chart specifies "perforation", which is similar to G(d), i.e. passage of the projectile through the target plate (either in condition to burst or broken up). However, "critical velocity" is actually the velocity where 50% probability of success is expected, quite unlike the G(d) definition, which prescribed 5 out 5 consecutive successes in a narrow velocity band.
The apparent drop of penetration at normal vs german RHA is really only reflective for the higher tensile strength grades involved in this thickness region. Similarely, at very deep thickness, the german RHA tends to be little softer than british MQ armor. 50% curves should be slightly larger than german 100% curves, which is exactly what we see here. Give or take, the quality of RHA isn´t that much different, at least on a significant level of difference, apart from where softer / harder grades sets in in regard to deep or thin section thicknesses.

[Mobius]

Plus there's a slight difference to the US success criteria in that only 20% the shell mass has to pass through the armor.

[Peasant]

Most likely, I don´t see much of a difference. The british chart specifies "perforation", which is similar to G(d), i.e. passage of the projectile through the target plate (either in condition to burst or broken up). However, "critical velocity" is actually the velocity where 50% probability of success is expected, quite unlike the G(d) definition, which prescribed 5 out 5 consecutive successes in a narrow velocity band.
The apparent drop of penetration at normal vs german RHA is really only reflective for the higher tensile strength grades involved in this thickness region. Similarely, at very deep thickness, the german RHA tends to be little softer than british MQ armor. 50% curves should be slightly larger than german 100% curves, which is exactly what we see here. Give or take, the quality of RHA isn´t that much different, at least on a significant level of difference, apart from where softer / harder grades sets in in regard to deep or thin section thicknesses.

You think? Then why the difference between the curves at 30° is pretty much constant while at 0° is not? And the british MQ RHA had very similar average hardness level (285 vs 291 BHN) in this period:

Plus there's a slight difference to the US success criteria in that only 20% the shell mass has to pass through the armor.

I don't think this would've made any difference for german 7.5cm PzGr.39 since they stay intact under these conditions anyway, and they either completely pass through the plate or don't, no X% mass threshold here.

[Peasant]

I think I've got it: using the information I have on the resistance of british RHA in the time period when these charts were made, I can suggest an explanation for this riddle.

As we can see from the chart posted previously, minimal thickness tested was 50-70mm or a bit more, at this point the target of given thickness and BHN spec would still show a good resistance to "penetration"(british standard) (albeit it might spall often on non penetrating hits, lowering it's protection value) at 30° against 75mm AP shells, but against the same attack at normal it would be well outside the optimum range of properties and its resistance would drop considerably, explaining the asymmetry in the chart.

Edit: In summary: because the penetration curves must be continuous while the BHN specs for the each range of thicknesses are given in a specific range and not as a continuous function of plate's thickness, the actual resistance of British RHA should be expected to be slightly better in live ballistic trials, in the 80-120mm range and slightly worse at lower thicknesses and 0-30° obliquity of attack.

[Peasant]

I thought that comparing the performance of 7.5cm PzGr.39 against german and british RHA might give some insight on respective nations penetration criteria. Seems like british plate behaves poorly at normal incidence when the T/D ratio is close to 1. I've checked the '45 chart as well and the difference between them seems minimal.

I’ve overlaid and colored the British penetration curves for the US 75mm M61 shell and the German 75mm PzGr.39:



And here is the german data for their shell:



There is a surprising difference between the british and the german data. According to German chart, the 7.5cm PzGr.39 should reliably pen only 100mm/0° at 650m/s, but the british chart gives 50% penetration limit at 2132fps of approximately 113mm/0°.
I wonder if this difference is entirely due to different penetration criteria or the british RHA is just that bad.
Because according to this chart here, the M61 APC should pen about 102mm/0° at that velocity, pretty reasonable amount when compared with penetration of the german shell.

The answer was actually much simpler than I originally thought:

Tell me, if you were in the position of Allies who just captured some of these german 75mm guns, what is the first thing you’d like to know about them? You are probably thinking: their armour penetration, right? Well, kinda. It would be interesting to know, of course, but what you are interested in the most is: “how much armour your tanks will need to be safe from this gun?”.

I’ve crunched some numbers to see the relationship between how much armour a gun can perforate and how much thickness will provide full protection, and here are the results:



So, a gun that will pen 100mm/0° will be stopped completely by about 111mm of armor.

Funny how these are almost exact numbers we see in german and british charts respectively. You know what I think? I think someone in the Ordnance office made a huuuuge mistake and used the values that describe the “safety thickness vs velocity” in place of “perforation vs velocity” when making this chart. The match is too good to be just a coincidence.

EXCEL file.

[Peasant]

In the UK technical jargon, the “ballistic limit” is the concept called “Army limit” in the US. While what they call the “ballistic limit” in the US aka limit velocity (meaning the numerical value itself) brits call “critical velocity”.
This is why I think its not too far fetched that somebody made a mistake in that british penetration chart for the german 7.5cm projectile.

[Timber]

I doubt there's any mistake in the charts, as AFAIK they're based on test results.

Also 75mm M61 behaved poorly in comparison to German 75mm PzGr.39 in US testing.

Finally Germans used a 2/3rds criteria IIRC, i.e. 66.6% of rounds fired must penetrate completely (100% of projectile found on other side), for all guns above 50mm caliber and above.

[Timber]

It's also interesting that similar British charts for the 8.8cm PzGr.39 show worse penetration up close than German data (192 vs 203mm @ 100 m), but better at long range (139 vs 132 mm @ 2 km). Indicating they were firing 8.8cm PzGr.39 from before the new German acceptance standards introduced in mid 44 in order to solve issues with older 8.8cm PzGr.39 breaking up at the much higher impact velocities imparted on them by the new KwK/Pak43 guns.

[critical mass]

I doubt there's any mistake in the charts, as AFAIK they're based on test results.

Also 75mm M61 behaved poorly in comparison to German 75mm PzGr.39 in US testing.

Finally Germans used a 2/3rds criteria IIRC, i.e. 66.6% of rounds fired must penetrate completely (100% of projectile found on other side), for all guns above 50mm caliber and above.

The Germans employed no 2/3 criteria. Actual definition of the G(D) for projectiles between 50 and 203 mm calibre required five out of five complete penetrations (100% of the shots fired had to completely penetrate within a thin velocity band).
within 0 and 30 deg obliquity, these projectiles also had to be passing in intact condition fit to burst.

[Timber]

The Germans employed no 2/3 criteria. Actual definition of the G(D) for projectiles between 50 and 203 mm calibre required five out of five complete penetrations (100% of the shots fired had to completely penetrate within a thin velocity band).
within 0 and 30 deg obliquity, these projectiles also had to be passing in intact condition fit to burst.

Hmm... coulda sworn I read they used a 50 & 66.6% penetration criteria. But either way, if what you're saying is correct, then my point stands even stronger as that would make the German criteria for a "penetration" even more stringent than the Soviet "80% of shots with 70% if shell weight on other side of plate" one.

Thus the British charts make perfect sense, and I see no reason for there to be any mistake in them.

[Thoddy]

I’ve overlaid and colored the British penetration curves for the US 75mm M61 shell and the German 75mm PzGr.39:

M 61 chart from ADA953368 Handbook of Ballistic and Engineering Data for Ammunition. Volume 1.

relative performance of M61 vs 7,5 cm pzgr 39
as far as i interprete the charts
performance chart of pzgr 39 is convex up to 1100m/s
whereas the M61 chart became concave above ca 500 -600m/s
so the M61 starts to deform at considerably lower velocities