In response to your request for a mini Super Cannon II:Silence wrote:People have suggested something that slides along the length of the barrel. Hard to do though.
McMaster-Carr's numbers say that LRT will expand to 8x its rest width and 3x its rest length.
Anyway, there are other things you could try if you're looking to do something unique. Making a spherical bladder (using balloons) hasn't been documented very well. Or maybe an air chamber with a piston inside (basically a mini-Supercannon II), so you can reduce dropoff to CPS-type levels? A unique shape, better ergonomics, etc? A well-balanced onboard reservoir? There are lots of options that might be more viable than a double-ended bladder, although I agree that the latter would be interesting and unique.
PRESENTING: THE SUPER CANNON MINI!
There will be either one or two sealed rods, if two connected, coming out of the back to pull the piston back into place after shots, with a mark on them when to stop.