It is mentioned in both Japanese and English-language materials. Here is an example excerpt from the report of the American naval technical mission:
Spoiler
There are other sources, but I don’t see the point in looking for them additionally
Technically impossible, until the Gaijin abandons its self-invented formulas for armor penetration (distorted formula from AaG), or until it makes an exception for Japanese AP shells, as for example it made an exception for Russian APFSDS which are configured manually by developers for the official reason “their characteristics cannot be displayed by a formula”.
It might seem strange to you that even late Japanese tank shells do not carry armor-piercing caps, while the technology was known to them (in the Navy, they probably created the most advanced, and the most complex in design/production armor-piercing caps in history). The fact is that Japanese armor-piercing shells have complex gradient front surface cementation and hardening, therefore they have a kind of “built-in armor-piercing cap”, what in reality is a technology that fully replaces a separate armor-piercing cap according to the results of ballistic tests, at least until we do not take battleship AP shells, where both technologies should already be applied due “square–cube law”.
Spoiler
Note - the hardness numbers are from an early projectile, but they demonstrate the hardness distribution well.
During the war, Japan was one of the world’s largest producers of tungsten thanks to deposits in Korea and Manchuria; in addition, through a network of front companies, China actively sold tungsten from its southern deposits to Japan. Having fairly decent access to tungsten at that time, in response to the increase in armor protection of enemy tanks, it was decided rather quickly to begin introducing tungsten into the composition of projectiles. This was also facilitated by limiting the growth of the power of guns. Due to the characteristics of the theater of operations, namely huge logistics lines, often extremely rugged terrain with undeveloped infrastructure, etc., the possibilities for increasing the mass of guns were limited. For example, the project of a 57 mm (anti-)tank gun, which was supposed to replace 47 mm (anti-)tank guns, was buried because, according to test results, operators found it difficult to operate it manually in difficult conditions due to the increased mass.
Spoiler
57 mm AT gun
By the way, for the reasons described above, they were quite attracted to APCR shells, but the APCR shells made using German technology turned out to be not much more penetrating than the later Japanese tungsten-chromium AP shells, even when shooting at vertical armor plates, and APCR did not go into mass production. As an alternative, German conical guns were studied (in addition, Japanese independent research on conical guns was going on in parallel), but ultimately they were also not considered promising, compared to the recoilless rifles.
When Japanese tank shells in production switched to from “regular” projectile steel to tungsten-chromium steel, the impact strength and hardness of the projectile increases by a quarter compared to conventional projectiles, which cannot be reflected by the current formula.
Spoiler
Note - Japanese shells contained 1% tungsten, as in the graphic materials provided. For projectiles, the hardness numbers (on the surface) would be even much higher due to carburization, the presence of additional alloying impurities in the composition and “hard” face hardening, but this will give a general understanding of the effect of introducing tungsten into the composition of chromium steel.
Pay attention to the improvement in the structure: how much the metal crystals have decreased, the structure has homogenized, and solid-phase precipitation has decreased when tungsten is introduced into the composition of the alloy:
In other words, neither the gradient hardness nor the unique chemical composition of Japanese projectile steel can in principle be reflected by such a primitive armor penetration formula without introducing any external coefficients into it. As far as I know, at least some developers are aware of the problem thanks to some assistant proactive guys, but if we translate developers words from “official”, they not really interested in messing around, this is not considered something important for the game. The funny thing is that the original AaG formula, taken as the basis for the gaijin formula, was closer to the reliable armor penetration of Japanese shells, everything was radically distorted by the gaijin’s personal “improvement of the formula.”
