Slitherine

Hi,
I have tried to edit Infantry Unit loadout: I have added a 2nd weapon to a unit of grey hunters, editing Units.csv using Notepad++ (I copy/pasted stats of the meltagun from grey hunters meltagun). I am running a skirmish to try this out and when I order the unit to fire with the 2nd unit that I added, nothing happens, I just waste one action pont.
What am I to edit to make sure 2nd weapon work in game?

Also, wehre can I find the files referenced under AssetFilename field of units.csv?

Thanks,
Giltharin