Let's say that in the Virginia Tech incident, machine guns were legal to CC and Virginia Tech allowed CC? Let's say that some student happened to have a legally owned machine gun in his backpack. Let's say it was belt fed and his backpack had a separate compartment for all the ammunition ready to feed into his personal defense device. After Cho shot that many innocent people, let's say that this student with the machine gun fires on Cho stopping the threat immediately and one of the bullets strays and hits a bystander? Wouldn't a lot of people be thanking this one student saying that he saved their kid who was also in the room who could have died, even though this bystander was shot? What legal and media fate would happen to this student who just saved perhaps a hundred students plus the 32 (minus Cho) from dieing?
That is a lot of what ifs. I suspect if your hypothetical machine gun carrying student used an ordinary handgun, he would lessen the chances of hitting that bystander in the first place than if he used his backpack mounted belt fed machine gun. Plus it would be lighter, less bulky, and easier to access from a holster. And you could carry books and a lunch in your backpack.