New Weapon?

[eftertanke]

As far as a practical use for a grenade launcher, there really isn't one.
To be fair, with the small, arcade style maps that AC players favor, an RPG/RL would only result in pissing people off.
It would be to powerful a weapon in my opinion.
Cube had RL's. So does Sauer.
It's just a sniper rifle with a bigger smoke trail and an explosion at the end of it.
In a nutshell.

It's just aesthetic. Not an RPG, but a grenade launcher attachment for the AR.

What's the difference between the pistol we have now and a combat pistol?

The combat pistol has different ammo amounts (I don't remember them of the top of my head), burst-fire capabilities, and is stronger.

EDIT:

Three-shot burst on the AR and SMG is a fantastic idea. What key would it be bound to?

It would probably be a toggle.