Much like I enjoy using my Uzi mags in different guns, I’ve been looking for ways to use my Galil mags in other platforms. Galil mags are interesting because they fit standard AK-47 magwells. So, if you’ve got a gun with an AK magwell, you’re halfway there.
Well, an “undocumented feature” of the Sig 556 platform is that if you put a Sig 556 upper on a Sig 556R lower, the gun will function just fine with Galil mags. There’s a small caveat there – polymer magazines won’t fit – but steel mags are just fine. I don’t know if this an improvement on the standard Sig 556, but it’s a clever hack.