I’ve finished one of the secondary air chambers and I’m in the process of making the main one. One of the main holdups was finding a way to trigger the chambers correctly (one is at -1ft, two is at 9ft and three is at 19ft from where the potato is).
The solution at this point is to fire them all at the same time and hope that the potato is going fast enough that little air escapes before the potato passes the last injector.
Update: This news post describes the original plans for Mark II. I’ve since redesigned it to use three valves all at the rear of the gun, which eliminates the timing issues I talk about above.