Many years ago, when running deer with dogs was legal in Texas and shotguns with buckshot were the prefered weapons, I came to some conclutions that I feel are valid for self defense purposes as well.
First is that buckshot is best used at very close range.....preferably under 25 yards or so. It can be effective at longer ranges but even the best patterning guns begin to spread the pellets pretty thin at more than 25 yards. For deer or self defense you want every pellet to strike in the "kill zone" and that means a spread of no more than 8-10". It matters not how many pellets strike in a 30" circle as many (maybe most) will strike "fringe" areas and not be effective killers. This is not a problem at all for typical self defense use as it would take an extreemely big room to offer a 25 yard shot inside a home.
Second, since the effective range of buckshot is so short, pellet size matters little. The only real advantage of larger pellets is better penetration at longer ranges. At close range any pellet size has plenty of penetration and multiple hits are more effective than any energy advantage a larger pellet might have.
Third is that most shotguns will pattern smaller size pellets better than larger sizes. This is particularly true if the choke is tighter than IC. In fact, with #4 Buckshot the very best patters are often had with a modified or even full choke.
I found a load of #4 Buckshot to be much mopre effective for me as deer medicine.....and would choose the same load for self defense purposes. You also gain the advantage that the lighter #4's will be less likely to penetrate multiple walls endangering unintended targets.
