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.
The absolute best part of the Firearm Blog post is the speculation about what 7.62x51mm gun it fits when there’re several pictures of it next to an M16 magazine and they have nearly identical outside dimensions.
I didn’t read all the comments, but one reason I mentioned those points is because I saw some people speculating way out into left field with what it could be. The pictures tell you things, like you mentioned.
I’m leaning towards the SPIW idea but I still think my concept of a Cold War era 30 Carbine AK conversion is more fun to think about!