Good try, but wrong.
Penetration is not linearly dependent on shell’s effective mass.
Let’s have a look at DeMarre’s equation:
Mass of the cap and windshield does not take part in the penetration, but those of the filler and the fuze do.
So, taking the effective mass of an APCBC shell as 85% of plain AP of the same total mass, we get that it would have (0,85)^(1/1,4) = 89% of pen of the AP, at low obliquity and under conditions where AP does not suffer shatter.
It’s complicated. Yes, some lots of ammunition had heat treatment defects, but the difference between pen of good quality 76mm M62 and, let’s say, 75mm PzGr.39 does not have anything to do with that.
There is no single big reason, its a combination of small things:
The windshield on the american shell is much bigger and heavier, giving it better ballistics but less mass for the penetrator itself.
It has a slightly heavier penetration cap.
Its tip is rounded/blunted instead of sharp.
All these things combined result in somewhat lower pen.
I suspect US APC caps were large for face hardened armor. The T50 had a much smaller cap and the result was a significant increase in performance from the same shell weight.
What’s odd is that they actually changed the reference values for both 15mm and 14.5mm APCR rounds, so that they have more or less historical performance instead of being nerfed by the calculator to be practically useless over regular AP.
So there’s pretty much no reason why Gaijin can’t do the same with other rounds.
They just need to a couple of things:
Well it could also be that those rounds could have incorrect characteristics (much like M304, M332 and T44 APCR rounds for the US 90 mm cannons).
However, I don’t know what stuff like the core weight and diameter of these rounds would actually be in real life. You might, however, so I’ll provide those values. In game the MG 151 15 mm APCR has a 9.5 mm, 38 gram core, and the 14.5 mm APCR has a 10.7 mm, 37.3 gram core.
For 15mm it was 9.5mm and 39g (38g might also be correct)
14.5mm was something like 10.9mm (maybe 10.7mm) and 38g.
So in-game values are pretty accurate.
Ok so I looked at the de Marre variables that are inside the M304 APCR round and compared them to the 14.5 mm APCR and yeah they are different. Specifically the “PenetrationK” value, which is 0.85 instead of 0.65.
However I have no idea how they work. And it doesn’t help that the APCR calculator on the wiki doesn’t actually match the penetration values shown in the game.
90 mm M304 APCR
14.5 mm PS-41 APCR
Hard to say. You would think that since 0.85 is 30% greater than 0.65, it just increases the performance by 30%.
Before the change the ammo had 39mm pen. Times 1.3 we get ~51mm.
So that’s pretty much it. They increased the performance by 30% to give it historical performance.
More on this topic: here I’ve overlaid and colored the British penetration curves for the US 75mm M61 shell and the German 75mm PzGr.39:
Looks pretty cool.
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.
Edit: The curves of M62 APC and M61 APC from TBVol.3 match well at 0° and 45° but deviate between themselves at 20° and 30°. Strange.
The US pen for M61 APC shell at 2132fps is 3,75in(95mm) and that of the M62 is 99mm.
Very cool. Nicely done :)
Isn’t that data more or less calculated? Basically extrapolated from some inital value.
There’s also some discrepancy between the British and German 30° values.
German values are 100mm at 0° and 80mm at 30° while British values for 30° are 75mm.
So maybe it’s because of a different 30° modifier, that the German shell received more 0° performance than expected.
But just looking at the difference in penetrator weight (+ a bit extra for superior German APC) would result in the 75mm Pzgr. 39 having around 100mm penetration when the M61 has 90mm.
Which would result in ~114mm when the M61 has 102mm.
But the M61 penetrating 90mm at 100yd would be against 240 BHN armor, afaik, while the Pzgr. 39 was tested against 300 BHN armor, which would explain the differences.
That’s because the M62 penetrator is slightly lighter, or slightly larger (caliber) for the same weight.
At least I came to this conclusion at one point, comparing the two rounds.
So the M62 would have slightly less performance at higher velocity compared to the M61, less noticable against sloped armor because the values are just closer due to the slope modifier not being visible.
As always, you are overthinking stuff, my dude. No, its not about plate hardness either, besides, changing BHN from 240 to 300 does not significantly change the perforation limit for a 75mm AP vs 100mm plate:
Since the penetration of the M61/M62 shell (they likely used the 76mm gun with M62 shell, because the data extends up to 2600fps) from the british chart almost exactly matches that of the M62 chart from TB. Vol.3, there must be something else at work here than “bad british armour” or “different penetration criteria” at work.
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.
If I remember correctly, the solid lines on the graph are from testing and the dashed lines are extrapolated.
If I understand you right, the TBV3 charts are more like protection limit than Navy limit?
That doesn’t make sense. As long as the plate is thicker than the caliber, increasing the hardness results in more resistance to penetration because hardness counteracts the piercing mechanism of the shell, which is most effective at 0°.
I guess nose shape also place a role but I guess we can discount that when talking strictly about the M61 and 75mm Pzgr. 39.
True. It says so on the page.
Good catch, I didn’t even see that.
Well, clearly this is not the case, as the NPL formula is not just something purely theoretical, but derived from real life experimental data:
As to “why” this is so: unlike what you’ve told me, the plugging type failure of the back of the plate doesn’t immediately stop as soon as it reaches a certain thickness, be it equal to 1, 2 or any calibers of the attacking projectile. If you play around with the calculator you will see that resistance of the plate grows with hardness, up to a point, and that point is where I believe it’s failure mechanism switches from purely ductile to plugging. But this point is located not at some fixed T/D ratio but is higher for thicker plates (relative to the caliber of the attacking projectile).
@MiseryIndex556 No, the limits for US data from TBVol.3 are Navy limits. It’s these british curves for the german 75mm projectile that are immunity curves, C/D limit as British call it, it is equivalent to the US Army limit (not the Protection limit).
Honestly yeah but Early and Late look ugly in names, so I would suggest M82/44 and M82/45 for the early and later variants respectively. Similar to how the Panzer IV has Pzgr 39 and the Panther has Pzgr 39/42. Obviously these names can be changed by introduction or something else but even if its as simple as M82/E and M82/L I would really like to see the words Early and Late kept out of names because it just doesn’t look good
Could this be true? In the section where the resistance of the T-54 is compared against NATO tank ammunition, they talk about the possibilities of the 90mm cannon against the T-54, and it says curious things.
