Shootem like this.
This youngish boar was moved into the light for photo, but it is a good example for instant incapacitation by a .223.
The side shown is also the exit wound, the other side the pig was slightly quartered away, so the shot was slightly higher and further back...maybe 45 deg from the ear. I show this picture because it shows a wound better than most pictures that I have taken.
50 yards, 55gr jhp, I don't recall the brand. I haven't used it again because it made an exit.
As you can see the neck is a pretty large target. If the animal is positioned correctly, it is an easy shot to make.
I believe the pigs fall so quickly due to hydrostatic effects, not necessarily wound diameter.