Well, I have made some progress, and am happy with the mold. I did several test castings, and to be honest none of them are exactly what I wanted to use in this application.
1 FlexFoam-It V is cool, but forget trying to make anything out of it with any sort of surface detail. Works great for Nerf foam balls, and fake scenery, but stinks for this applications.
2. Foam-It-8 is a cool product, and I can see lots of uses for it, might even cast a lightweight blaster from the stuff, but, I expect these 'Bunker Busters' to be tossed around a lot, and stuck on cars, doors, and such and these do break pretty easily. [I tested one with my 5 year old. Did not survive a few tosses] If we want to do some feather weight ones, this can be used for sure, but can not be tossed around or it will break easily.
3. Smooth-Cast 65D is an awesome material. Very resistant to chips, and breaks, and may become my new 'go-to' resin for general casting. Down side is, that even hollow, it's still heavier than the foam version.
4. Smooth-Cast 45D is another awesome material, and it can take a dye, if we wanted to do these in black. It is also very resistant to chips or cracks.
5. Feather Light. I am still testing this. Cure time is a bummer, 2 hours. Strength is not yet determined. Weight is still unknown.
Here are some pics of the test so far: