Here is the Instructable for my fourth heavy cannon. It is much even smaller and more piece efficient than the last, and still keeps a good range of 65-70 feet.

A few things to point out...
I removed the wheels. They were great, but used too many pieces. There is now a stand that is much more piece efficient, and still gets the job done.

I added a carrying handle and disguised it as a sight. It works as a sight as well.

I managed to greatly simplify the trigger, making it a lot easier to build, and is easer on pieces.

Ammo has stayed the same as the Nano's.

I made a great change to the slide. Instead of the white connector of the bottom of the slide being on the bottom slot of the yellow connector, it is off to the side. This makes the slide shallower, and therefore greatly reduces friction. See picture seven to see that.

There it is! I hope you like it. I hope you can build it!

I have made a few other versions of this gun, check them out!
Original
Mini
Nano
Handheld

Step 1: Parts List

Alight. This is the parts list. I managed to cut down quite a bit from the Nano. I hope you gots the pieces!

FOR THE CANNON:
Connectors
Dark Grey (one pronged)--26
Light Grey (two pronged)-8
Red---------------------------19
Green------------------------46
Yellow-----------------------187
White-------------------------16
Blue---------------------------28
Purple-------------------------64
Orange------------------------47

Rods
Green-------------------------148
White--------------------------108
Blue---------------------------107
Yellow------------------------20
Red----------------------------29
Grey---------------------------6

Other
Medium wheel----------------4
Silver Spacer----------------24
Blue Spacer-----------------48
Y Connector-----------------6
#64 Elastics------------------17

FOR ONE AMMO
Rods
Green------------------------14
White-------------------------6
Connectors
Red---------------------------3
Green------------------------4
Blue--------------------------6
Purple------------------------2

Got all those? Lets get started!

Step 2: Stand

This is the stand. Sorry, not wheels. They used too many pieces. This is effective though. I hope you like it.

1: The stand in question
2: Make this part. There are 6 rods.
3: Get these parts
4: Make the panels
5: Put the red rod through the white as shown
6: Attach the two thingys on either end
7: Put the part you made first on the bottom like so
8: Finished stand

That was easy!

Step 3: Upper Barrel

This is the upper barrel. BORING!
Yes, well, just build it.

1: The barrel in question
2: Make this part
3: Close up of the joints
4: Close up of the end
5: That whole part upside down
6: Build this part (sorry about the fuzz)
7: That part the other way around
8: Make these two guys
9: Put them on that last part like so
10: Connect that to the rest of the barrel
11: Finished

Time for the lower barrel! ALSO BORING

Step 4: Lower Barrel

This is the lower barrel. I think it might be identical to the upper barrel. (Hmm, I should check that)

Anyway, lets build this thing!

1: The Lower Barrel
2: Make this set of track first
3: Close up of the joints
4: Close up of the end
5: That whole bit upside down (actually, I guess its right side up now)
6: Make this end bit
7: That part upside down
8: Make these two parts
9: Put them on that last piece like so
10: And connect the barrel like so

Yay! Those boring steps are now out of the way. Onward!

Step 5: Bow

Welcome to the bow. I hope your build is both easy and enjoyable.

1: The bow
2: Make this one part first
3: Close up of the slide
4: That part upside down
5: Make these two parts
6: What they look like from the side
7: And from the top
8: Make this part. It is the other half
9: What one side should look like. Remember the spacers
10: Close up of the slide
11: From a angle so you can see the vertical rods
12: That part upside down
13: Close up
14: Close up of that yellow connector
15: Slip the wheel attachments over the bow like so
16: What that should look like

We are now going to string the bow. Later, the other half of the bow makes this much harder, so we are going to do it now. Be sure to thouroghly stretch the elastics before putting them on.

17: Take two elastics and place them like so
18: Pull the one underneath through so it looks like a cross
19: Pull the other elastic through so it looks like so
20: Pull tight
21: Place another elastic over the one on the end
22: Poke the other elastic through
23: Pull the other one through
24: Tighten
25: Put one more on the string like you did the last one so it is four elastics long
26: Make three more strings
27: Take two elastics and wrap them around the bottom wheel
28: Hook it onto the blue rod as shown. Make sure that they are at the same height as the wheel.
29: Pull it around and hook it on the other rod like you did the last
30: Put the other two strings on the top wheel. It will bend a little, but that is ok
31: It should look like this on top
32: Now attach the top half og the bow on top
33: Make sure all the vertical rods are connected
34: Finished bow!

YAY! YAY!

That was a long step. Good work!

Step 6: Trigger

This is the trigger. Thankfully, I managed to make this much simpler than all the previous triggers. It uses far less pieces too. Yay!

1: The trigger
2: Make this bit
3: That bit from the top
4: From the bottom. The rod can be yellow
5: Make this bit. There are 10 greens
6: Make this little piece
7: From the back
8: Yet another angle
9: Put the ten greens on top of that last piece like so
10: It should look like this
11: Put that part you made first on like so
12: That part you just made. Set it aside for now
13: make these bits
14: Make this part
15: Now make this part. The following four pictures are of this one piece
16: ---
17: ---
18: ---
19: ---
20: Put that box thing on the back like so
21: Now put one of those yellow/red pieces on each corner like this
22: Put that white part in the back
23: Put the knob on top
24: It should go like that
25: That rod goes in those holes
26: Finished trigger

Whoop whoop, thats done. Booyah!

Step 7: Add-ons

This is the step for all the little pieces scattered all over the gun. Though they may sound optional, most of these are required for the gun to work properly. The scope can be used as a carrying handle, just so you know.

1: This is the scope/handle
2: Just make the top bit for now
3: That from directly above
4: And from directly below
5: Make this base part
6: Put the two together to get the finished part
7: Make these parts. They will end up on the bow
8: What they look like
9: Close up of the end
10: And of the other end
11: Make these four people

Thats out of the way. Time to put this together!

Step 8: Assembley

Time to put this cannon together. This is the fun part.

1: All the bits you should have
2: Start by attaching the stand to the bow
3: Close up of that
4: Attach the lower barrel like so
5: Close up of that
6: Put the upper barrel on now
7: Close up of that
8: Now put the trigger on (sorry about the blur)
9: Close up of that
10: Invert the whole thing and put the longer of the two barrel supports in
11: Close up of the end
12: Close up of the front
13: Put the scope on. Easy
14: Put the shorter barrel supports on like so
15: Close up of the end by the trigger
16: It attaches to the scope
17: Put three of those arm-like things on each side
18: Close up of the one end
19: And of the other
20: Put a elastic on the trigger like so. It should not pull hard, just enough to make it snap into place
21: Assembled Cannon

Only one thing left to do. Learn to shoot!

Step 9: Ammo, Loading and Firing

Finally there. Loading can be hard, but with a little practice it can become quick and easy.

1: Start my making these parts
2: Put the two on opposite sides
3: The other two on the top and bottom
4: You got your ammo and your gun
5: Pull the bottom two strings back like so
6: And hook them around the connector like so
7: It should like like this
8: Pull the top two and hook them like so
9: Now it should look like this
10: Insert ammo in the front
11: Pull back the knob so that the rod in the trigger is out of the way
12: Put the red connector on the back of the ammo into the line of connectors and push the trigger rod through.
13: Take the elastics from their locks and put them on the back of the ammo like so
14: Aim...
15: FIRE!

Now you know how to fire your cannon. Yessssss.......

I added a video of the whole loading process.

Loading 016.AVI
Download

Step 10: Finished!

Yay! You are finished! Congratulations. Be sure to use your cannon with care. Please. I am not liable for any damage caused.

Thanks for building!

I_am_Canadian