Go have a look on the Firearm Blog. It’s got me stumped. Sometimes it helps to list what we know has to be true:
- From the front lip and nub in the back, this magazine gets rotated into place. That should speak to an AK style magazine function. That rules out anything with a straight mag well.
- The overall length of the cartridge is less than that of a 5.56x45mm cartridge, but from the follower and width, would be wider.
- It would likely need to fire a straight walled, or fairly straight walled cartridge, because otherwise the magazine would need some curve or bend.
I have no idea what that could be to, but it’s definitely obscure.