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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 ?

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You've probably found by now that the best answer to "not do it!" The undermounted shotgun, also known as the "masterkey", is a pretty awful weapon. It's weak and unless you get the drop on someone at close range, trying to complete the attachment's achievement can be an exercise in frustration. – Bernard Dy Mar 21 '13 at 0:06
up vote 2 down vote accepted

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.

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Nice one. Thanks for that. – François Wahl May 26 '12 at 0:19

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