If you want a unique airsoft gas blowback, here is the Zaku Mobile Gundam Suit Gas Blowback Machine Gun. All you need to do is get the Action Army AAP-01 GBB Pistol and buy the C&C TAC ZA Style Kit to do the conversion. Mach Sakai does a conversion so he can check out how it performs at his range.

Gundam fans might want to take a look at this project. The Zaku Machine gun gets replicated and it's a working, shell-ejecting model with fire modes from single, 2-round, 3-round, 4-round, to full auto and the Pan magazine also works. I cannot confirm if this is an airsoft one or more of a cap gun-type since it shows some smoke when fired.

But what got me interested is at the beginning of the video in which an airsoft gun with a pan magazine actually works.