1. order "hold fire"
2. oder "hold this position"
3. once your troops are where you want them order "fire at will" at your leisure.
the problem with gun mods, and ranged troops in general, is that they will carry out this annoying shoot-move-reload-move-shoot maneuver, which plays havoc with your formations. so the first thing you should do when you spawn during a battle is order your troops to hold their fire, then order them to hold position where you want them to be, i.e. riflemen on a hilltop.