Hi, this is my first instructable Hope it works for you =)

This is my K'nex tank. It has a gun that fires, it can drive both backwards and forwards, and it is sturdy.
(It fires air soft BBs)

Pros: can drive, can fire, is sturdy, looks really good, and tracks don't come off very often

Cons: Turret doesn't rotate, and there is no mag :( but you can add one...

Step 1: Begin the Body

Build these. These pieces I'll call #1 and #2

Step 2:

Now build these... These are pieces 3,4,5,and 6

Step 3:

Now these... these are pieces #7 and #8

Step 4:

Put pieces #7 and #8 together and put a motor in between...

Step 5:

PART ONE: OK, now add three grey rods and put a grey spacer on each rod (both sides)....

PART TWO: now put pieces 3 and 4 on (shown in image 2)

PART THREE: now add another grey spacer on each rod and then put on pieces 5 and 6 (image 3)

PART FOUR: now add these spacers onto each of these rods (image 4)

PART FIVE: now add pieces 7 and 8 (image 5)...

Step 6: Putting on the Tracks

PART ONE: now put these on (image one)

PART TWO: put a grey spacer on each rod THEn put on wheels then a blue spacer (image 2)

PART THREE: build these. These pieces are #9 and #10 (image 3)

PART FOUR: put pieces #9 and #10 on, one on each side (image 4)

PART FIVE: now put on the tracks! =)

Step 7: Building the Gun!

PART ONE: build this. This is the firing pin (image 1)

PART TWO: build this. This is the gun mount (image 2)

PART THREE: build this . This is the trigger (image 3)

PART FOUR: now build this. This is the gun barrel (image 4)

PART FIVE: build this. (image 5)

PART SIX: build these. there are ten (image 6)

Step 8: Building the Gun! Part II

PART ONE: now put the gun mount on the circle piece like this... (image 1)

PART TWO: now take the trigger and attach it to the barrel like this... (image 2)

PART THREE: now take the barrel and attach it to the gun mount like this... (image 3)

PART FOUR: now take those ten pieces that you made in step 7 and attach them to that circle piece that has the gun on it with blue rods... (image 4 and 5)

PART FIVE: see those two blue rods in image 6? attach them to those two black circle pieces in image 7...

PART SIX: the gun is on!!!!!!! (image 8)

Step 9: Elastics!!!!!!!

OK, here is how to put the elastics on here is a picture of the back of the gun (image 1). Now put the firing pin you built in step 7 in the barrel in the back. Like this (image 2). Now put a strong elastic on like this (image 3). Now put another elastic through the hole and put on two blue rods like this (image 4).
(The more elastics you put on, the more power so, add as many elastics as you like. Just don't over stress the gun.)
You are done this step!

Step 10: How to Fire

To fire, you need to pull the firing pin back far enough to be able to push the trigger up, then load your air soft BB into the barrel from the front. Now push the trigger down!!!!!!!!

YOU'RE DONE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

<(o_o)> :D :p :) (x_x) <- LOL