Summary

Needed more power, so made longer barrel for this elastic band gun:
http://www.thingiverse.com/thing:603697/#files

Instructions

print and use in place of the shorter barrel.

22-03-15
Added barrel coupler.
Slots over the end of your existing barrel and lets you extend the length by slotting another barrel into the other end.
For some super powered elastic band shooting.

Print your barrels with at least 10% strong infill - otherwise they might well bend under the pressure !
I'd recommend 20% plus.
I printed the coupler with 3 layers and shells and 10% infill at 0.3mm resolution.

Oh yeah and stand the coupler on end to print.

And if you are daft enough to use it with the coupler and both barrels - be bloody careful loading small elastic bands. So far I've only loaded 2 at once. It starts getting scary past that point :-)

Also added a short barrel specifically for use with the coupler. So you can have a variety of barrel lengths.
Also using the coupler and short barrel you can adjust the length of barrels in your slicer safe in the knowlege that the hole shape and size doesn't matter.
Basically lets you use ANY length barrel you want to without taking the gun apart to change barrels or having to design a new length barrel every time you need a different length..

Longer barrel(s) for 6 shooter elastic band gun