"Cardboard Kevlar" - FACT or FICTION? Can plain old cardboard stop 15 calibers?

[barnbwt]

It takes a lot of cardboard to stop a 7.62x51, but strangely enough, the hollow points didn't expand well after passing through.

I'm not an expert in terminal ballistics, but it seems like expansion/energy transfer in cardboard would be very different from tissue since the densities of the materials are so different. I wonder if this is why several of the rounds mentioned failed to expand (that is, they dissapated their energy by breaking cellulose fibers instead of by plastically deforming).

TCB