Board Thread:Questions and Answers/@comment-4928331-20120918112046/@comment-4928331-20130206030431

You can do the physics yourself. How much kinetic energy is a bullet packing in order for it to knock a 165 pound man 10 feet backwards, considering the small surface area of the bullet tip? Now, apply that energy going the other way, coming from the gun. I don't care how tanky you are, that gun is going to conk you on the nose at the very least.

That's why it's a Hollywood myth. Mythbusters also tried to replicate Batman's grappler, and his car's hook, with interesting results.