It is obvously pneumatic, on their site it says that it has a max operating pressure of 250 psi, which isn't to be sneased at.
panzerfaust
If you look carefully at the video you will see that he had a compressed CO2 cylinder on the back of it, the same type that paintball guns use, which is how he got such a heigh rate of fire.
http://www.sanchospaintballs.com/yah...123414_std.jpg
To make it I imagine that you could modify a paintball gun to make it more powerfull, and of coarse a bigger barrel. The trigger mechanism looks prety simple in the video.
I couldnt give you any specifics on how to do this because I know nothing about painball guns, sorry. If you figure this out please post it.
Ps, I have an incling of a feeling that the CO2 is fed into a chamber first, before it enters the barrel ( 0:05 - 0:10 in the you tube video), maby it has 2 modes of fire; range, where CO2 is feed into a chamber first, and rapid fire where the CO2 goes directly into the barell. I dont know, just a thought, my minds in a jumble at the moment.