Req: What kind of mag is that?

[A3OT]

Hi guys,
I just discovered this picture on Google and it kinda confused me. The magazine of that Ar15/M4 right in the middle looks way bigger than normal magazines. Could anyone tell me more about it?

http://www.abload.de/img/defense-4m3np.jpg

[Sir Zach of R.]

Looking at the barrel and seeing that it's blue, it looks like it's a 9mm conversion upper receiver that fires 9mm simmunition.

[LineDoggie]

Sim Mag, just turned a bunch back to my old Armory after going through some duffles