.50 with a rotating band?

I’m currently on Guadalcanal scouring the area. I recently came across this cartridge. Given the location, this is WWII era ammunition. Any ideas?

I measured the base of the cartridge at 17mm and the 20mm at it’s largest diameter. The projectile itself was never recovered and I regret to say that I didn’t take any additional measurements. It’s been driving me crazy trying to ID this round. Any help would be greatly appreciated.

Looks like a 20mm Ore or H.S. proj. to me. Either of which could be War II, but I’m sure others will know better.

Yes, definitely a 20mm, not a .50 BMG. Off-hand I can’t think of any US ground or aircraft applications in that caliber during 1942, so if that vintage, probably from a US Navy ship or a Japanese fighter. Always possible that it was left during US late- or post-war training.

Japanese 20x80 Aircraft Oerlikon ammo Projectile. Widely used in Guadalcanal by Japanese Bombers and Fighters…they even had a flexible version (waist/Observers gun).

Doc AV