That has been the mystery... they supposedly were tripod(actually bipod not tripod) mounted. He alegedly had 10 guns in the room.

Though one possible thing other than chain fed (military specialists think it sounds chain fed) is that it was semi-automatics with a crank mechanism attached to them that you can crank to make it fall faster and that will also give a little bit of the varied velocity sound that can also happen a bit with chain fed.

A lot of info. Still no clue which of it will survive.