My remake on VIK's design. I made het handle longer, the tolerances a little tighter and holes in het handle where you can put some spare foam darts.
I added the 123D-file so you can change it if you want to.

Best is to see the instructions from VIK.
Mount the two parts with two M3 bolds (or some 3mm filament)
Add rubber bands
Put a piece of wire trough the hole as a spring for the firing pin