Not a lot of pieces.
NOTE: you need 2 grey and one black, these are the long ones. The firing pin is black. Stronger.
I used a small part from KILLERK's gun, the rest is my idea, if your gun is similar, keep in mind I didn't get any ideas other than from KILLERK.
Step 1: Make the Handle.
Two yellow rods with red connecters on them.
Step 2: Make the Firing Pin.
A black rod (not in first picture) with 3 orange connecters on the back and 1 on the end.
You need to use black (stronger) unless you want it to shoot only 1 foot.
Put tape on the back to strengthen it.
Step 3: Make the Barrel.
Use the grey in the back to keep the handle from coming out.
Step 4: Insert Firing Pin.
Like in the picture.
Step 5: Add Rubber Bands.
The picture says it all.
Step 6: Fully Cocked.