20mm penetration data - a question

[JasonC]

I've noticed that the Russian 20mm L82 has 97.4% of the muzzle velocity of the German 20mm L55 but the later has 37% higher maximum penetration numbers. This seems a large difference, and I was wondering about its cause. I can think of plenty of plausible explanations that might make it right - like APHE vs AP - but I am wondering what the actual rationale was.

I've seen figures all over the map for both guns. For the Russian, I've see 28mm at 100m flat, 31mm with AP, 24mm with APHE, statements that the penetration equalled the 37mm, others saying as low as 15mm at 300m without specifying the angle. For the German, I've seen 31mm at 30 degrees, 31mm flat, 28mm flat, and 20mm at 30 degrees - all for 100m range.

These numbers seem to routinely vary by a factor of 1.5 from source to source, and sometimes by a factor of 2. Which makes me wonder if they know which gun, ammo, distance, angle etc is actually being discussed in each test, as opposed to simply imagining that "20mm penetrates this much" is the whole story.

In CMBB game terms, the difference is most apparent when Pz IIs go up against Russian T-60 light tanks. The T-60 turret is not as well protected as its other front plates, which are comparable to the protection of the Pz II. But the real difference is that the T-60 gun drastically underperforms the Pz II gun. The Russian 20mm seriously underperforms the 14.5mm ATR.

I know that air combat sims model the Russian 20s as at least as good as German 20s of the same era. But it may be that reflects APHE performance rather than AP vs. AP. Can anyone shed any light on the subject?

[John D Salt]

Originally posted by JasonC:

I've noticed that the Russian 20mm L82 has 97.4% of the muzzle velocity of the German 20mm L55 but the later has 37% higher maximum penetration numbers. This seems a large difference, and I was wondering about its cause. I can think of plenty of plausible explanations that might make it right - like APHE vs AP - but I am wondering what the actual rationale was.

I've seen figures all over the map for both guns. For the Russian, I've see 28mm at 100m flat, 31mm with AP, 24mm with APHE, statements that the penetration equalled the 37mm, others saying as low as 15mm at 300m without specifying the angle.

I would hazard a guess at it being a question of the relatively low mass of the Russian round, but I can't find what this should be anywhere.

Tony Williams' "Rapid Fire" (Airlife, Shrewsbury, 2000) gives the following details for the Russain 20 x 99R and German 20 x 138B cartridges, which I believe are the relevant ones:

20 x 99R HE

Projectile mass 97 g

Muzzle vel 860 m/sec

Muzzle energy 35.9 kj

20 x 138B HE

Projectile mass 119 g

Muzzle vel 900 m/sec

Muzzle energy 47 kj

20 x 138B AP

Projectile mass 147 g

Muzzle vel 795 m/sec

Muzzle energy 46.5 kj

While the lack of information on the TNSh AP round makes direct comparison impossible, the fact that the HE round of the German gun has about 30% higher muzzle energy suggests that something similar might apply to the AP rounds.

Originally posted by JasonC:

These numbers seem to routinely vary by a factor of 1.5 from source to source, and sometimes by a factor of 2. Which makes me wonder if they know which gun, ammo, distance, angle etc is actually being discussed in each test, as opposed to simply imagining that "20mm penetrates this much" is the whole story.

You're doing better than me, then -- I think I've only seen two sources that give penetration performance for the TNSh, and one for the ShVAK, so I'd be interested in knowing what yours are. Sources that are completely precise as to weapon, ammunition, range, angle of strike, nature of target and penetration criterion are, as I;m sure you know, rarer than rocking-horse manure.

All the best,

John.

[JasonC]

Thanks for the reply.

I think you are right about the basic difference, that one is a x99 length round of 96 grams, and the other is an x138 length round of 148 grams. That mass difference neatly coincides with the ratio of CMBB penetration values against vertical plate at 500m. It still leaves as a puzzle various reports that put the penetration of the German gun significantly below the CMBB level. But it is plausible CMBB has just been more careful than a lot of such sources and got it right.

One table with low info for the German round is here - http://www.panzerworld.net/APT.htm

Notice it has a round of 148 grams with a MV of 780 m/s, which matches the value for the latter used by CMBB. But it gives 20mm penetration at 100 yards vs. 30 degree angle, way below CMBB performance. There is a possible explanation of that, however. The 20mm penetration number is about what CMBB gives to the HE. So maybe these lower numbers are for the wrong ammo type.

Notice the much higher value for APCR with 1050 m/s MV but only 100 grams shell weight. Also the complication of an L112.5 20mm, FLAK rather than KwK. I've seen HE-I rounds for FLAK with 900 m/s as well, but with the gun unspecified. CMBB gives the MV of the HE as 880 m/s.

Then here - http://www.wwiivehicles.com/html/germany/guns.html

You see what is apparently the same data as above being given in the first 2 lines, for the 20mm Kwk 38 L55. Then there is another pair of numbers, and it is unclear if they are meant to be from another source for the same gun, or from the FLAK as opposed to the KwK. The second of them gives for APCR essentially the same number as the previous. But for AP, it gives 31mm at 100m at 30 degrees, 22mm out to 500m. Much closer to the performance CMBB gives to the German gun. Perhaps the difference is two sources, and although they don't know it those sources differ in the number because one is actually talking about HE-I and the other about AP-I. (Note that "HE" is something of a misnomer with these tiny rounds. It is a question of a 6 gram burster being present or not).

On another site I saw the 20mm at 30 degrees and 100m number repeated, with source given (rare and useful) as "Salt's snippets" - you? It seems that CMBB believes that figure is correct only for the HE, not for the AP.

Now look at the CMBB figures. The Russian gun gets 22mm vs. 30 degrees at 100m, very close to the former number of 20mm for the German gun; slightly better actually. The German gun gets 29mm vs. 30 degrees at 100m, marginally worse than the latter number for the German gun.

Meanwhile the same site, discussing the T-60, says of the development issues mating the 20mm gun with the chassis - "Initially it was designed with a 37 mm gun, but this proved too powerful for the turret. B. Shpital'n was given the task of coming up with a powerful gun for the tank. He developed the rapid firing 20 mm ShVAK-20 gun. It could pierce as thick of armor as the 37 mm could penetrate."

Which is obviously very ambiguous, and not terribly believable. The 37mm L45 B-3 of 1931 is given 27mm at 500m, without an angle specified. The German 20mm in CMBB gets 25mm vs. flat plate at that range. For Russian 20mms, the same site gives -

http://www.wwiivehicles.com/html/ussr/guns.html#20SchVak

two line entries, an L62.5 Schvak and a L/107 TNsH, neither of which says it is the L82. The former has a listed MV of 750 m/s, close to the CMBB 760 but not exact, and 24mm at 100m without an angle specified. The CMBB 100m values straddle that, 27mm vs. flat and 22m vs. 30 degrees. The longer Russian 20mm is given 22mm flat at 500m, specified APHE, which is 37.5% better than the CMBB value of 16mm.

For the gun in the T-60, I have seen it specified as the TnSh-20, called a slightly modified version of the ShVAK aircraft main armament gun.

The BZ round was API, 96 grams (fits your data), 750 m/s MV which is slightly under the CMBB figure, but matches the site above for the L62.5 rather than the L82.

There are 3 kinds of BZ alone - a tracer, and another described at "Mild steel projectile case with hardened steel core, surrounded by 2,5g incendiary, screwed on aluminum, or bakelite ballistic cap.", with a alternative "Solid steel shot with incendiary in swaged steel cap". The OZ round is listed as 770 m/s, with 2.8g HE burster and 3.3g incendiary, a lighter shell of 75-6 grams - that sounds like an air combat and AA round (small burster, incendiary). The CMBB MV figure is the mean of those two.

I wonder if the CMBB figures are modeling a compromise between BZ and OZ. I'd assume the AP in tanks would be BZ - they weren't shooting at planes with them.

Overall, though, I puzzle it out as follows. The 50% lower figures sometimes seen for the German 20mm are correct for the HE-I, rather than the AP-I. The AP-I is a much better penetrator. The MV is not higher but actually lower. It out-penetrates the Russian 20mm despite similar MV because it is a significantly longer and therefore heavier round.

The only thing I have left I am confused about is the 38% variation in some of the Russian numbers, and the confusion over the Russian gun differences, L62.5, L82, L107?

Thanks for your help and prompting. Looks like CMBB probably got this one right.

[ January 24, 2003, 05:22 PM: Message edited by: JasonC ]