How do I use the Undermounted Shotgun attachment for assault rifles?

In call of duty: modern warfare 3 (PC version) when playing multiplayer. You can select at weapon level 20 the shotgun attachment for the assault rifle.

How do you fire it?
Does it happen automatically or do you need to use a hot-key ?

To use an undermounted weapon attachment (either a grenade launcher or a shotgun) you'll need to activate it - this is similar to switching weapons, although it's generally a bit faster than raising or lowering a secondary.

I believe this key can be configured, but by default it is 3. Once you've switched to the undermounted attachment, fire it just like you would any weapon.

Default key for PC is b: it shows up when you select the weapon.