On the last version of this gun we had two 2x4's mounted vertically directly behind the tee that the barrel mounts to. We tried to clean it up as much as possible on this one, and with the ratchet straps tightened fairly tight, it seems to transfer the energy to the frame fairly efficiently. It hasn't moved in the frame and since these water shots are probably the heaviest thing we'll shoot we figure that it what we have now works fairly well.
And yes this barrel is ridiculously short, and we can use barrels two or three times as long as even our longest barrel we don't have a way to transport them. Also, after seeing what this thing can do with even the shortest barrel im not sure making the barrels that much longer is really a good idea
Hopefully goin shooting tomorrow, we'll try to get some good vids as well as pics of the longer barrels.