"Ideal" 9mm bbl length?

[MPfiveengineer]

Ok, noob questions here;
If it's the same round, what is the difference? Is the longer barrel on the MP5 getting more velocity out of the round or stabilizing better or what?

More velocity, kinetic energy = 1/2 (mass)(velocity)^2

What's the deal with "keeping a bullet subsonic"?

When shooting with a silencer you need the bullet to stay subsonic so that it does not make a sonic crack when it comes out of the silencer.