ORIGINAL: idunno
The size of the patern at the ranges necessary in personal protection would only be around 4"or 5" maybe a little more. I'm talking about inside an apartment or residence where there may possibly be innocent bystanders. Your neighbor in the next apartment with only a couple layers of sheetrock between you for example. Overpenetration is a definite concern. another thing to look into is the total fragmenting rounds like RBCD
http://www.rbcd.net/gelatin%20photos.html They are designed to expend all of their energy into the target without exiting again reducing the chance of an injured innocent bystander in my case my 22 month old and 5 year old sons in their bedroom. Just another example
The problem is that anything that is acceptable for home defense (to STOP a bad guy) needs to have sufficient penetration, otherwise it is insufficient/useless. Frangible ammo (designed to work in a body, but stop better when it hits the consistency of a wall) is a good step in the right direction-hopefully it will become a proven HD option, I don't know if it is right now or not, but seems like a good idea in theory.
What difference does the size of the pattern make? If you have no combined mass or sectional density (bunch of tiny BBs, bunched up , but independent of each other), the penetration will suck regardless. Need at least a 20 gauge with #4 buckshot, IMO.