Slugs punch big holes in things no different than a pistol bullet. It's just a big damn hole punched by a 1 ounce projectile that will easily smash large bony structures.
Buckshot as typically used in the woods patterns like dogshit and so a typical hunter puts a couple of pellets dispersed widely on the animal which is a lot like shooting it with a couple of poorly performing 9mm rounds.
Putting a proper pattern of buckshot on the animal is an entirely different phenomenon because all the pellets hitting more or less the same area at the same time produces a
dramatically different result.
Here's a video that demonstrates what I'm talking about
:
https://youtu.be/VNUCc1WvYxA
The long range shot of 65 yards hits the animal and it falls, flops, and eventually runs off to bleed out.
The medium range shot at 35 yards drops and does the funky chicken on an incline, enough for gravity to move the animal a few yards.
The close range shot of 13 yards...which is pretty close to realistic self defense distances...anchors the animal
instantly. At that distance the pattern was likely no bigger than a fist (probably not much bigger than the bore given the hits he was making at longer ranges) and if you were to dress that animal you'd see that the organs around the impact area had been essentially turned to soup and then the pellets went on to cut individual wound tracks as they spread out.
The physiological design of four-legged creatures means that most shots fired on them do not impact their CNS absent some deliberate targeting. The CNS of the human animal, on the other hand, is right there behind the stuff we're trying to hit to shut down a threat. So if you are given a full frontal profile of a threat and fire a shot into the upper thoracic at typical engagement ranges with a buckshot round that provides a good pattern, it is going to punch through the sternum with ease, pulverize the inelastic tissue in the area, shred the elastic tissue in the area, and after that as the pellets spread through the anatomy at least a couple of them are highly likely to impact the spine and/or very large nerve branches coming out of the spine.
This proves highly effective at stopping whatever action that guy was taking that made you press the trigger on him in the first place.