Small hole, high velocity.
That's the point. It's a .22 caliber slug at very high velocity. The piercing power is extreme.
Unless you have atypical ammo the bullet broke apart.
It very well might have, but not before it went through a quarter inch plate of steel. I shoot FMJ exclusively in the Mini-14's. No hollow points.
We were discussing home defense though, not many people have 1/4" thick steel walls. Box of Truth or something like that, did the test with multiple sheets of drywall. The AR round was the worst for penetrating down the line.
Dry wall is soft, it doesn't take much to go through it.
My point is a 9mm is a fairly large slug, with not a lot of grains of powder - about half of what is in a .223 load. So a slug less than half the mass, propelled by over twice the powder; the velocity is exponentially greater.
When I was a teen in the 70's, "Guns and Ammo" did a series on the trend of police switching from .38 specials to 9mm. The main reason was that the .38 had a tendency to go through walls, car doors, bodies, etc. and hit what was on the other side. They wanted a firearm with similar stopping power due to slug mass, but less muzzle velocity. The 9mm fit the bill.
A 9mm has nowhere near the power of a .223