have you considered something like:
Patient Marksman: Special action: If you have not exited your space during this activation perform an attack, add one blue dice to your attack pool. This attack gains Piece 1 and +1Acc.
Id suggest that for the elite card (remove piece and acc for regular?). It gives a bit of a theme of a sniper (set up, wait then take shot) but doesn't hamper them too much should you need to move first as they can still use their 'pistol shot' (normal attack pool). There is also a precedence for this sort of thinking in the game if you look at the Rebel Troopers card.
If you like this idea I'm sure it would also warrant a points increase as only one action for it might be deemed too cheap even with the no movement prior limitation
Edited by Collins