'Shotguns have dkb/dbt' - where did that myth (?) come from?

[Anthony]

Originally Posted by lwcamp

The 7 kg arm of a karate master punching at 7 m/s will have 170 J of kinetic energy and 49 N-s of momentum.

Um...wow. 7 kilogram arms? I think you're vastly overestimating the weight of the arm, particularly the amount of the arm which is actually moving at peak speed; based on how much water I displace when I put my arm in a container of water, the forearm is maybe 1 kilogram. For reference, knife-proof vests are generally normalized for a 40J impact, which is more than most people can actually deliver.

Of course, knocking someone back with a punch also pretty much doesn't happen, unless they fall over from the shock. About the only impacts which realistically cause any level of knockback are collisions or other similar attacks which involve the full body weight behind the blow.