RE: knockdown power... fact or myth?
The force itself is not going to knock over an a big game animal, if that's what you're getting at. However, the force, expansion, damage it does to vitals organs and breaks bones obviously can and does knock down large animals often. So, yes the characteristics of the slug, powder charge, force,resulting in shock to the animal, significant damage and blood losshas everything to do with bring the animal down fast and for good.
That's what matters, not whether a slug "knocks over" the animal.