M320 Grenade Launcher: Under-slung vs. Standalone

I cannot find a definitive source for this, but apparently:

• 40mm flechette rounds are unlocked after ten 40mm grenade launcher kills

• 40mm smoke rounds are unlocked after twenty 40mm grenade launcher kills

Why this can't be displayed in the battlelog in some form along with all the other zillions of weapons unlocks, I have no idea. But this is what people reported from the beta.

edit: confirmed, by me personally getting the unlocks. I presume the same rule applies to the Support class mortar, get 10 kills with mortar to unlock smoke mortar rounds.

As you already guessed, underslung attachments replace your primary kit, just like the normal grenade launcher would, which would be, as in the example of the assault class, the medkit.

You switch to your underslung attachment by pressing what you normally press for the medkit, you will then switch to your grenade launcher. You can verify that by observing the slightly raised barrel stance of your rifle.

If you haven't chosen the underslung rail as attachment option, then you will access to the grenade launcher as a separate gun, which will cause it to take longer to switch to and back.

For more information, take a look at this Wiki page.