Step 1: Getting the Basic Shape
Next I put in the magazine, because lets face it, it'll look like a rifle no matter what as long as it has a cool looking magazine sticking out. The first step for this was to use the L-shape. Play with the thickness, width and length until you get an L-shape that is about as wide as you want the final magazine to be and not quite as long. Use the "Tweak" feature to elongate the shape some, increasing the 90 degree angle to somewhere about 130 degrees. Then use the "Fillet" tool to round the inside and outside of the now V-shape. It'll take a little bit of experimenting, but you should end up with something that resembles a magazine.
Once you have the basic shape of the magazine, rotate it and move it into the center bottom of your rifle body, and at this point it's already starting to look like a rifle!
Step 2: The Handle
Step 3: Adding Features
Decorative block: I added a block, the same width as the body, to the top of the rifle and then rounded its corners, just to give substance to the body.
Trigger cover: For the trigger cover I used "C-shape" that I used to "Tweak" feature to skew a little bit. Then I rotated it and intersected it with the handle and body.
Forward Grip: The forward grip was nothing more than a block I put over the bottom of the barrel and then added fillets to. Once again to get an idea of the size I gripped a ruler to see how long it should be.
Trigger: To make the trigger, I took an "L-shape" and skewed it with the "Tweak" feature. Then I added fillets on the inside and outside corners. (Adding the trigger was purely for fun and demonstrative purposes. When assembling the cardboard, the shape of the trigger won't be maintained. The trigger will have to be part of the detail work after the basic shape has been assembled)
Step 4: Dressing It Up
Once you've done any tweaking you want to do, you're done with the 123D Design portion of the tutorial!
Step 5: Design Template
*Note* The barrel is only intended to help with the design process. A cardboard barrel isn't going to work very well. If you want to use one, go for it, but I'm going to cut off the cardboard barrel, and insert pvc pipe instead.
Step 6: To Be Continued...
Step 1: Insert the barrel. I want the barrel to be more rigid than cardboard, so I plan on cutting out the cardboard barrel and inserting pvc pipe into the cardboard body.
Step 2: Smooth out the edges. Take some clay or putty or bondo, or whatever you have lying around that might work, and coat the cardboard with it. This is intended to fill in the "steps" between each layer of cardboard so that the shape is smooth and rounded.
Step 3: Add details. Next use the clay to add some knobs and switches. The small details couldn't be made with cardboard (or at least not easily), so we'll utilize our clay instead.
Step 4: Paint! Use whatever paint you have. I'm planning on using spray paint for a base coat, and then using brushes to add details, such as a logo and switch covers etc.
Step 5: Enjoy it! Go use it in a movie, go trick or treating with it, or give it to a friend. Just don't get into trouble with it!
Sorry I couldn't finish the instructable yet. I hope I was able to give somebody a good starting point. Enjoy!