The really awesome thing about this build is at the con you feel like you're in the game. Most prop guns don't show in your field of vision but this thing sitting on your shoulder looks exactly like what you see in game which makes being a solider even more awesome.
4.5" diameter PVC pipe
2" thick insulating foam
0.5" thick insulating foam
24x36" sheet metal
1/8" craft foam
small piece of 1/2" wood (for the handles)
1/8" wood or other solid material
lots of hot glue
Step 1: Preparing the foam
The tip of the rocket launcher is the easiest to start with. The pics are for the wooden bit, but the steps are the same for the two parts. Cut three 6"x14" pieces of 2" foam and glue them together. A simple hacksaw works well on foam, plus you can put it at an angle and make some long cuts. Go easy on the glue, the more you add the harder it will be to cut the and the less round your final piece will be. I usually go with one light coat on both sides; let the glue dry for a minute then press the two halves together with some bricks or something heavy. If you're worried about the foam flying apart on the lathe, don't. It doesn't hurt that bad when the pieces hit you. :)
When turning foam, always cut the pieces as wide as your foam is stacked. This way you can make a piece as close to round as possible before putting it on the lathe. It's going to be rough to turn this down to a circle and the closer to round it is starting off the less likely it's going to fly apart or break.
Cut the corners off the glued together block so you end up with an octagon.