From another thread:
Polish Taipan: Good question is could you get grendel ammo to work in an AK magazine? Just off the top of my head I would think the 5.45 mag would work best, but I forget the oal of the grendel. I'm not sure what thread its in but Templar suggested a milled reciever for a grendel based AK. I like the idea. I may start another thread to see if we could get a concept rifle going. Photoshopped concept of course. (At least at first).Ordananceguy: Nice! I'm still kicking one of those cartridges around, in my head.
Grendel ammo should work in the AK mag, with mods. The base case is the 7.62x39, but the taper was taken out of it. So would that make it closer to the 5.45? I don't have a Grendel case to try out.
Edit: Grendel OAL is about the same as 5.56 and 6.8 SPC, all to fit in the 5.56 mag length. Grendel bullet seems to be seated further out. 6.8 SPC mag has shallower flutes in the mag sides, to accommodate the fatter cartridge, compared to 5.56.