As you may know in the late 1950s two anti-aircraft gun systems were developed in the USSR, both based upon the 500P automatic cannon by Alexandr Nudelman: the self-propelled ZSU-37-2 Yenisey (with twin guns) and the towed  system known as the MKS-107 or Shkval with quadruple installation. Soviet/Russian sources (mainly works by Alexandr Shirokorad) state that the gun in question was not a development of any previous Soviet anti-aircraft 37mm guns and it used ammunition than had nothing to do with the 37x250R used earlier.

Googling around for info on the guns (and their ammunition) I found a Russian forum (apparently for cartridge or military artefacts collectors https://guns.allzip.org/topic/216/1868764.html) where a guy was (actually in 2016) offering a cartridge that according to him was nothing else than a cartridge that had been used in the 500P cannon. He said the cartridge case was 220 mm long and it had a 'ZTN' (Zapalnaya Troobka Nordenfelda or Nordenfeld primer; frankly I do not know which type of the primer the Russians call that way).

Below photos of the MKS-107 Shkval and Yenisey and obviously photos found in the Russian forum. You may see how fat the cartridge is. Clearly it does not resemble any other (known) Soviet ammunition.

Of course, the guy from the forum said nothing about how he knew it was a 500P cannon cartridge and where he got it from, so his post must not be regarded as a credible source. Nevertheless it seems to be an interesting clue what the ammunition for the 500P might have been.

Piotr

Projectile having been actually fired:

Thanks, that's interesting. Any more information about the cartridge (case diameters, weighs MV etc)?

I have seen a suggested link between the Yenisey cartridge and the 37 x 240 Chinese (current naval ammo).

In sources on the ZSU-37-2 Yenisey there is info on ballistic perfomance of its guns: projectile weight 0.733 kg, MV 1010 mps.

RoF of the Angara system (twin 500P guns) in the ZSU-37-2: 1048 rpm (so 524 rpm for a single cannon). RoF of the Shkval 4x600 rpm.

Frankly I tried to gather info on the ammunition for the Yenisey/Shkval just because I suspect that the Chinese somehow took advantage of the 500P cannon and/or its ammunition. There are some Chinese projects (eg. BJ-212 jeep, J-8 fighter, Luda-class destroyers etc.) that strikingly resemble projects that were cancelled in the USSR. The Chinese might have been given the whole projects for further development or they 'acquired' them a different way (and then reversed engineered them). Possibly the Type 76 naval gun (and 37x240 ammo) was 'inspired' by the Soviet developments of the late 1950s.

Piotr

That's interesting. The PLAN 37 x 240 is credited with firing a 725 g pro at 1,010 m/s, which is VERY close to the Yenisey.

NORINCO has offered a couple of mountings - the original H/PJ76F naval twin mount in 37 x 240 (RoF 360 rpm per barrel, apparently based on the OTOBreda Fast Forty / Bofors) and the much simpler "semi-automatic" AN3, available in 37 x 240 or 35 x 228 Oerlikon. This gun is based on the Oerlikon KDF and fires at 550 rpm. 

The pic shows a 35 x 228 round above a replica of the 37 x 240. The main difference in effect is that the 37 mm fires a heavier projectile at a lower velocity.