Fire At Will-Constant fire

[Voodo chile]

How do you make a unit have the ability to fire at will where everyone fires at random times so you dont end up with a volley.

Its for Napoleonic 2 total war and i want to be able to switch between volley fire and fire at will. I haven't seem anything like it in Rtw. And if it is possible how you do you do it.

If its impossible then i'll go for the next best thing which is 'fire by rank'

[Aradan]

Firing 'modes' cannot be altered in any way, and all missile units have them (you can't force a missile unit to not be able to fire-at-will).