The MIAD has two holes in it and one is further back from the one used for the AR-15. That second hole looks like it might line up with the SIG556 mounting hole. Could you guys at Magpul try it? Or at least tell us if it lines up?
I was going to order an extra MIAD to see if I could fit it on my SIG556. If I have to TIG weld then I will, but I'd rather not.