It's controlled by a regulated, external air supply. The input pressure can be increased or decreased to adjust the velocity.
The cost of the unit is far, far, eclipsed by the cost of the ammunition. 8mm is three times the price of 6mm, and that's conservative.
I dunno how much ammunition the M134 goes through in a game or two, but, let's ballpark it around 7,500 or 10,000.
In 6mm, that's only like, $25, if you buy Matrix BBs in bulk.?
In 8mm, that's $200+, buying .45s, and I don't think they are even available in bulk.
If this was in 6mm, it would be much, much, much cooler.