You'll need the pieces shown in the picture as well as 3 'standard' elastic bands or some reasonable equivalent. If the gun snaps often you can superglue the parts on. So let's get started!
*The trigger should fall down due to the elastic bands.
1 'hubcap' brown
1 black joint
2 light grey
2 black 'square' joiners
6 blue (4 when using revised version)
If in doubt all will become clear in the following pictures.
Step 1: Construct
Step 2: Build Firing Rod
Step 3: Finish Firing Rod
Step 4: Build Trigger
Step 5: Handle
Step 6: Main Barrel
EDIT: Please look at note (yellow box surrounding blue connector) the gun works better without this blue piece - sorry for inconvenience.
Step 7: Next Part of Barrel
Step 8: Finish Barrel
Step 9: Add Trigger
Step 10: Fix on Handle
Step 11: Gather Elastic Bands
Step 12: Fix on Elastic Bands
Step 13: Fire Away and Hold in Right Place
HOW TO FIRE
Pull back the firing rod a long way until the trigger drops down (if it does not, your elastic bands are either too weak or in the wrong place - manually push down the trigger in this case). Load your ammunition, aim, holding the gun as shown and push the trigger to the left. The firing rod will shoot at whack the ammunition which will fly out very accurately.
I use white rods with red 'arrowhead' connectors as ammunition but anything will work - I just found that these were the best. If the gun does not work for you please post a comment and I will try my best to help you out - the gun is usually very reliable and fires powerfully.
Step 14: Modifications
Thankyou for pointing this out.