I've been surfing gun boards of all types for at least 10 years, I grew up with guns - mostly shotguns but also .22's, I've shot a lot of things over the years and I have been around other shooting scenes.

First I have shot MULTIPLE animals with birdshot that did NOT die. I shot my neighbors chickens because when I asked my neighbor about keeping them out of my garden he said shoot them so I did. I shot at least 7-8 chickens with 20 ga. birdshot. Most of them didn't even notice it. Chickens. I also shot a rabid fox twice with that same ammo. It didn't flinch. Bird shot is NOT good for defense against humans no matter the gauge of the shotgun. Buckshot is devastating to humans. Birdshot may make a wound that looks awful but not penetrate the chest cavity at all. If it won't kill a chicken I ain't using it for defense.

I haven't investigated crime scenes but I know of one where a guy I knew shot up a trailer with 7.62 X 39 ammo from an SKS. It penetrated both sides of the trailer and an interior closet where it killed his intended target. Some ammo is made to penetrate.

Personally I much prefer .45 ACP to 9mm. You get about the same penetration but you get considerably more transferred energy with a .45. I've heard all the arguments about modern 9mm bullets. The thing is those same principles are used in modern .45 caliber bullets.

I have a video of a comparison to terminal ballistic performance between a 9mm and .45. The chart and the channel clearly show the .45 delivers more punch with minimal more penetration. And the test is not done with 230 gr. ammo like it should be. They show smaller rounds and then draw the erroneous conclusion that the terminal ballistics are the same even though their own chart shows it isn't. The larger .45 round delivered significantly more energy to the target and a 230 gr. shell would have delivered even more. They used two weights of .45. The lighter round, 160 gr., was about equal to the 9mm. The larger 180 gr. bullet was not. It was more powerful clearly. Just as the energy transfer was higher with the 180 gr. round it should be even higher with a 230 gr. round.

People will argue that a .45 is harder to handle. It is but if you have the right pistol it is minimal. I have a .45 that is very easy to handle and one that isn't. The axis height of the barrel affects this issue a lot.

As for lighter bullets in a 5.56 all I can say is I've killed feral dogs by shooting them in the head with a .22. I've "never" failed to kill one with a single shot to the head. That doesn't mean a .22 is just as effective as a larger round. It isn't. The path to a vital organ is just very short with a head shot. If you have to penetrate some obstacle like an arm or you hit a door frame first your bullet is going to fragment too soon and not penetrate far enough. Heavier weight bullets will do better in that situation. As you already know your backstop for your bullet will matter more than anything else for keeping your bullet at home. And there is a post from someone that should know that bullets exiting a house are rare unless you're using a round that will penetrate a 16" tree like a 6.62 x 39 will. I've seen that happen.

Also you have to consider the state of mind of a potential attacker. If you get someone totally out of their head on drugs or just psychotic an ugly wound may not deter them one bit. They may not even know they have been shot. You really need a bullet that will do the job or buckshot that will do the job. Both can be very effective if you make the right choices for bullet design or shell design and the power behind both. BTW someone suggested using a shotgun slug. If you want penetration that's the round to use. They transfer a huge amount of energy to whatever they hit.

Again here's the comparison test of 9mm vs. .45 ACP. Look at the chart yourself and see that what they tell you isn't true. BTW gel is a better predictor of tissue damage than almost anything else. It reveals a lot about the transfer of energy to the target including wound channel and penetration.