@militant: That is kinda what I'm getting at here. 9mm deflects more when it interacts with a barrier. That leads to poorer shot placement because the bullet doesn't go where you aimed it. However, 9mm also has less recoil and a higher capacity so it allows you the option to send a follow-up shot to compensate.
I'm not talking about helmets. I'm not talking about penetration. I never was. I was talking about defeating barriers with minimal deflection. Penetration means nothing if the bullet is no longer going to keep moving towards your intended target.
I know that helmets and body armor are literally "barriers" in the bullet's path. But, when people talk about barriers in the firearms world, they are referring to the things in the environment that tend to be between the shooter and the target (i.e. car doors, tires, windshields, sheet rock, wood, et cetera).
I didn't look through all of those tests. But, none that I saw had any information about the amount of deflection the various bullets had when encountering various barriers. Also, the only barrier I saw tested was windshields. I didn't look through all of the tests, though.
This isn't a very scientific test but you can see that .22LR, 9mm, .45 ACP, and .357 Magnum wouldn't penetrate a kevlar helmet even at close range. I know you were talking about steel helmets but I just wanted to throw it in there.
So, I'm not sure what you thought I was saying. I was trying to say that the more massive bullets larger calibers allow for, are less likely to be deflected due to their higher momentum. Less deflection means more of your bullets actually hit what you aimed them at. I'm not talking about penetration depth or ability to defeat helmets and body armor.
But yes, the data you presented is why I'm a fan of 9mm (even though I only saw .357 SIG in those tests), it has no problems with penetration at all. And, personally, I'm not going to try to shoot someone through a barrier if I can help it. But, the capacity and lower recoil of 9mm compensates very well if there is some sort of failure to hit them anyway.