First and for most I want to give credit to I_am_Canadian, whose cannons not only are some of the most terrifying things I have ever seen made from Knex but also inspired this monster.

Here are some of the specks:

weight: 4.4 lbs. (almost 2 kg)
range: with ten rubber bands around 35-40 feet (10 to 12 meters)
*I only have shot this with 10 bands but it is built to handle 20, I just don't have that many right now

Update-added parts list and a step for the ammo (which I only just noticed I forgot to add before)

Parts List:

Rods:
Black/Green- 209
Grey/White- 176
Blue- 197
Dark Grey/Yellow- 9
Green/Red- 21
Grey/Orange- 1

Connecters:
(1)Grey- 70
(2)Light Grey- 13
(3)Dark Grey/Red- 23
(4)Green- 33
(5)Tan/Yellow- 106
(2)Brown/Orange- 64
(8)Black/White- 64
Half snaps- 124
Full snaps- 49

Other:
Blue spacers- 83
Grey spacers- 13
Y-clips- 10
Lockers- 4
Hindges- 4

Step 1: Shoulder Rest and Rear Handle

This area can be used to hold extra ammo.

1. make this
2. and this
3. add the two X parts, the three blue rods, and the five connectors with greens
4. make two
5. make one
6. you need five of these
7. assemble the handle...
8. like this
9. trigger
10. add these three rods with Y-clips (start with the one in the white piece) ...
11. and this one
12. put the trigger in
13. add these, don't forget the spacers
14. attach to one side
15. mind the spacers
16. other side and more spacers

Step 2: Bow and Front Handle

Basically the front of the weapon that holds the rubber bands and the second handle.
Based on the bow from the hand held V5 cannon.

1. make this
2. and this
3. same as the other handle, all layers are the same
4. same as in the V5 cannon
5. add them to 2
6. make these
7. add them to the handle (add four blue spacers to these rods when you attach the handle)
8. attach handle (spacers)
9. add the bands, two strings of five on each wheel
10. put on the top

Step 3: Rails

Two sides of the barrel of the gun.

1. Make this,
2. and these,
3. connect like this,
4. and make it's mirror image

Step 4: Other Parts

Small parts that don't fit in any category.

1. make this
2. add connector, rod and spacers
3. make this
4. connect 2 and 3 like this
5. don't forget the small grey rod on the end
6. make these

Step 5: Trigger Block Assembly

Rear of the gun, holds the rear of the barrel and trigger mechanism based on that in the V5 cannon.

1. make this
2. and this
3. trigger block
4. back of block
5. make this
6. 4 and 5 assembled
7. make two of the rails and four clips
8. back of the rails from 7
9. assemble 6 and the parts from 7
10. top view of 9
11. add these supports
12. attach part from 2...
13. and the other side
14. add the trigger mechanism from step four*

Some parts were removed so you can see where the part fits in, leave them in.

Step 6: Assembly

Puttin' it all together.

1. all the parts
2. add the long piece to the trigger
3. these small parts go here
4. connect
5. add the rails
6. add the trigger block
7. remember to connect these!
8. DONE!

Step 7: Ammo

The orientation of the bow and trigger block components means that the standard ammo for the V5 cannon won't work for this weapon. Here's how to make the new ammo.

Step 8: Loading and Firing

How to load this monster.

1. pull the rubber bands back to here, make sure they go where they are in the picture
2. slide the ammo back and lock it into place like in the cannons, try to get the red piece as close to the end of the yellow rod as you can it will make firing easier
3. move the bands onto the round where they are in the picture

Then pull the trigger (outside please).