Right, this is my first K'nex gun: It goes pretty quickly, and can take lots of rubber bands (the maximum that i've got it to is 8 rubber bands)
Yes, I know that its a VERY simple block trigger - so please no comments saying "its not a true trigger!!"
It generally works best with around 4/5 rubber bands, and shoots anything up to (and including) yellow rods.

The Black peice that is at the front of the gun is the only supposedly "rare" part, and probably won't be avalible in a newbie set of K'nex. It can be replace with a beige peice, but it's less accurate.

Step 1: The Parts List!

What you need:

3 Red Rods
1 Blue Rod
5 Orange Connectors
1 Black Connector (unknown description)
13 Grey Connectors
1 Beige Connector

up to 8 rubber bands
Several bits of ammo (the white rods are best)

Step 2: The Rubber-band Holder, Exit Barrel and Trigger

First connect an orange connector onto a red rod, and a black connector behind that. (Picture 1)
Secondly, add 8 grey connectors to the red rod, and afterwards, slide an orange connector onto a the rod, and add the blue rod to 1 end of it. (Picture 2)

Step 3: Ram-Rod and Holder

First, you must add the remaining 5 greys to the barrel- this is the Ram-Rod holder for when the gun is cocked.(Picture 1)
Secondly, you must make the Ram-Rod itself - the Ram-Rod is made up of a Red rod, with a beige connector, followed by an orange connector on the end. Don't add this to the barrel quite yet, though. (Picture 2)

Step 4: The Support Rod

The Support Rod is to stop the barrel from bending when cocked, and so therefore will make the gun more effective and will prevent the ram-rod catching, for the main part.

The Support Rod is very basically a Red rod with 2 upright orange connectors

Step 5: Putting It All Together...

First, add the support rod that you made in the last step - the first orange connector goes at the front of the barrel, and the second one goes behind. (Picture 1)
Secondly, slide the ram-rod into palce and add the rubberbands by firstly hooking them underneath the black connector, then streching them back to the orange connector on the end of the ram-rod. (Pictures 2 and 3)
Make sure that one side of the rubber bands is OUTSIDE the trigger, so you have a self-cocking trigger.(Picture 4)

You now have a completed gun!!!

Any mods/improvements are welcome