Step 1: Tools
the only uncommon tool i used was a hand-held hack-saw my brother has.
other than that i used a Phillips head screw-driver, a pair of pliers and some wire cutters
Step 2: Lots of Hacking
then you'll have to cut away some of the casing. The parts that cover the barrel on the right side and and some of the casing where the gray pieces attach to it.
and the small pieces inside that are meant to hold those pieces in place.
sorry i didnt think to take pics of the barrel before hand and the casing shots are from my broken one
Step 3: Reassembly
Replace the other half of the case and screw it back together
Step 4: Notes
In testing i noticed that you have to hold down the button in order to get the barrels in and out.
While holding the button you can flick the gun and the barrels will swing through, one in one out, however they tend to not slide in all the way and you may have to jiggle it a bit to get it in place.
Again this was done on impulse with little planning any ideas on how to better the design are welcome even a better instructable if you try this mod yourself