After a while the bullets do begin to eat their way through it, and a .44 or .50ae will eventually blast through the back side, which can put on a little display of a splinter shower for the viewers!
Now, you want to see something explode, try fruits or gallon milk jugs full of water!
Also old hard drives and cd roms are pretty fun to shoot, they bounce around and fly apart pretty good, even with 'little' calibers like 40 and 45.