Recent pickup here with this .950 JDJ round which is about 24mm in diameter. For all intent and purposes this is a cannon round produced by SSK Industries. They only made 3 rifles to fire this round and have stop making ammo for it. Also enclosed is a side by side comparison with a 20mm Vulcan and a US 25mm.
https://www.military.com/video/guns/rifles/guys-shoot-the-950-jdj/1409750262001
https://www.military.com/video/guns/rifles/guys-shoot-the-950-jdj/1409750262001