OK, so why does a 5.56 mm at 55 gr stop so well.
It's a source of heated contention as to whether it does. The main reason, though, is that at velocities above 2700 feet per second, the bullet will often fragment, creating a larger wound area. This comes, of course, at the cost of penetration, something that is critical for the a battlefield rifle round.
You do indeed get hydrostatic shock in rifle rounds; it's why they make such nasty wounds, especially when they hit bone.
But too light of a bullet, even at high velocity, won't have enough energy to do it.
At the same token, a heavy bullet at too low of a velocity won't do it either.
As I said, it's a function of MOMENTUM and ENERGY, not just velocity. I don't know where you get this 1600fps number from. A 50 grain projectile at that speed will do a lot less damage than a 250 grain projectile, and a lot more damage than a 25 grain projectile. You need both mass and velocity to get energy.
And, from what I'm given to understand, slap a 135 grain bullet in there and you can easily get 1600 fps out of a 10mm handgun. It's gonna have some kind of muzzle blast, though.