Every once in awhile, there is a post by someone who had their PowerMac totalled in shipping. Usually, they are majorly pissed at one of the shipping companies, and post pictures of the results.
You might have some luck searching up one of those posts, I don't imagine the poor person has much use for the broken case afterwards.
My guess is that an El Capitan case might stop a .22 caliber, but nothing bigger. Please do not shoot at the Apple logos on the case, that would make the Mac-haters way too happy.