fmj actually penetrates less than buckshot and pistols too.

It's not skewed research - it's the FBI doing it to see if they are going to have higher liability inside buildings.

That's why you don't see mp5s all over the place any more with agencies that have nearly unlimited budgets. They've all gone to .223 powered carbines or short rifles.

You can do the testing yourself - I would have if I hadn't found overwhelming evidence to support my claims.

The theory behind the actual performance is that the bullet weight is so much less that it loses it's power much faster than a pistol or a round of buckshot when it encounters resistance.