I'm sure there were probably multiple stunt packs, but I can say for sure that Steve's jet pack is not rubber.
ETA - I have to amend this. I'm going to hedge my bets a little. Parts of the jet pack definately appear to NOT be rubber. There is at least one part on top that would get hit first, that looks like it could possibly be a hard rubber. You can see where paint is flaking off on most of the parts of the pack, and there's only one where something black shows through. The rest of the parts where it looks like paint is scraped down all the way, the pack doesn't have any black showing through, and seems to have a whitish looking plastic at the base. However, it could be that the rubber was soooo coated with something that would harden, and that is what is showing as the base. But, if that were the case, then I think more detail would be lost.