It's not even the blast from the cannon of the M1128 Stryker that scares me the most (though it is a very impressive blast). It's the fact that right after the 105mm cannon goes boom, the world around it looks post-apocalyptic. The ground shakes so hard from the gun that it looks like lost souls are being released from hell.
Here's video of it firing:
Which was edited from this longer video about the Stryker: