The shooting press and ammunition manufacturers talk a lot about gelatin studies and how they relate to defensive shooting. Greg Hamilton weighs in on the topic:

The main problem with the gelatin studies is that gelatin only is calibrated for pig muscle for depth of penetration. Not anything else. We can assume human muscle and pig muscle are similar enough that the is basically the same. After that all bets are off. The biggest problem is we shoot people in the chests, chests are mostly NOT muscle tissue. The measurements where the FBI came up with the magic 12-14″ are of the chest to transverse the goodies.

I wouldn’t shoot people with more than a 55 HP unless I was also trying to solve some other problem like distance at the same time.

Even though the street data is horribly collected, it is still data, and the best performers as “stoppers” on the street have been rounds that penetrate 8-10″ of ballistic gelatin. The FBI set 12-14 as a magic number and then went on to find rounds that go 12-14″ of muscle not 12-14″ of chest, which is the target they are concerned with getting through.

The 5.56 round with best real world results in the USA is a 55 HP, but in gelatin tests it only goes 10″ at most, yet it produces more “1 shot stops” than anything out there. Because it doesn’t penetrate far enough in gelatin people are discarding it but it has the highest DRT rate.

I wouldn’t shoot people with more than a 55 HP unless I was also trying to solve some other problem like distance at the same time.