According to this source, the cartridge of the 625 is 25 x 287 mm.Another thing, I looked up Chinese 25mm ammo (25×183mmB) and it looks like the ammo is only used in PLAGF. A quick search says it's a bored-out and longer-cased derivative of the 2A14's 23x152mmB cartridge (RU 23mm)
So my question is, why bother developing that ammo when there are similar ones out there? For example, Russian 23mm or NATO 25mm are both available, and China does use RU 23mm in the PLAAF (J-10 cannon) and most importantly in PLAGF (Type 85 Towed AA the ZU-23-2 copy).
To me, it feels like a waste of time and effort, especially when you already have comparable ammo in your logistic chain.
Even if it is not true and the 625 is using the 25 x 183 mm I can see reasons why they wanted to develop a new round.
- Make your caliber unique so others can't use it.
- Adopt a longer bullet for better ballistics.
- If you want to include a proximity fuse, 2 mm can be quite important.
- AFAIK all the previous Chinese 23 and 25 mm were shorter munitions with the exception of the semi-rimmed 25 x 218 mm.
If they developed a 25 x 287 mm cartridge, it means one thing. They want muzzle speed. If the 625 is really for C-RAM that makes a lot of sense.