I have had a lot of questions, summed up as follows: How effective is birdshot (#4, #6, #8, etc.) as a defense load?
We have done tests with various birdshot loads. Birdshot penetrated through two pieces of drywall (representing one wall) and was stopped in the paper on the front of the second wall. The problem with birdshot is that it does not penetrate enough to be effective as a defense round. Birdshot is designed to bring down little birds.
A policeman told of seeing a guy shot at close range with a load of 12 gauge birdshot, and was not even knocked down. He was still walking around when the EMTs got there. It was an ugly, shallow wound, but did not STOP the guy. And that is what we want... to STOP the bad guy from whatever he is doing. To do this, you must have a load that will reach the vitals of the bad guy. Birdshot will not do this.
In fact, tests have shown that even #4 Buckshot lacks the necessary penetration to reach the vital organs. Only 0 Buck, 00 Buck, and 000 Buck penetrate enough to reach the vital organs.
Unless you expect to be attacked by little birds, do not use birdshot. Use 00 Buck. It will do the job.
But doesn't 00 Buck penetrate too much in interior walls to be a "safe" load in a home?
Yes, it does penetrate a lot. But any load that is going to be effective will need to penetrate walls to have enough power to penetrate bad guys. If our only concern was to be sure we didn't penetrate walls, we would use BB guns. However, BB guns will not stop bad guys.
Therefore, we must use loads that will STOP bad guys, and this means that they will also penetrate walls. So, be sure you hit the bad guy and do not shoot into walls where loved ones are on the other side.