Supposidly the round is between a 5.56x45 Nato, and the 9mm. So you get the penitration of the rifle round, without the power.
So fast enough to go through the body armour, without going through them and the wall and killing someone on the other side.
The MP7 is a sick MF, I saw what it could do to a Kevler helmet, wow!
It looked like a 7.62 hit it!