Thanks for the comments regarding the use of these shells. I was thinking more about pressures, temperatures, and barrel abrasion when I posted the question. But their use in an auto brings in other factors that certainly need to be considered.

As for the validity of the experiment, I can't comment. Here are a couple of links to the issue. The second one contains a photo of some Carolina Bays. Note that this topic has been reported by National Geographic and the BBC.

http://space.newscientist.com/article/dn11909-did-a-comet-wipe-out-prehistoric-americans.html

http://www.andrewcollins.com/page/articles/Firestorm.htm

http://news.nationalgeographic.com/news/2008/05/080506-comet-extinct.html

http://news.bbc.co.uk/2/hi/science/nature/6676461.stm