On my blog I have some tutorials on how to make 2 part molds, as well as ways to use instant mold.
http://warfrog.blogspot.com/p/completed-projects.html
For the people wanting to make small items like heavy weapons definitely check out the Instant Mold option it will be the most cost effective way to got.
All of the heavy weapons and part of the backpacks in this photo we made with instant mold and a type of green stuff.