How can I create a weapon with a charge-up time before firing?

[alpha993 121]

I've recently started working on an assortment of fictional weapons and so far I have been able to figure out most of how the weapons config works and what-not.

However, one particular weapon is supposed to act as a kind of powerful single-shot cannon which I would like to have a charging time for. To elaborate, I would like the player using the weapon to have to hold down the trigger for say, two seconds, before it actually fires.

An example of the effect I'm trying to achieve would be something along the lines of the secondary fire mode of the X-PAC Mortar from Crysis 3. Here's an example video:

Any help on how to do this would be greatly appreciated (if it is even possible in the first place). Thanks! :)