If you have a gun on the flight deck you presumably need to be checked every six months on your proficiency to use it. It would be logical to do this with a sim check, and would probably best be done before the check in case relations become a little strained whilst in the box.
Would this refresher be a quick ten minutes potting sparrows on the roof of the sim block during a meal break, or would it be something more formal, say a range attached to the canteen using last weeks pies for targets.
Do you get graded on your marksmanship? Could you fail a check for poor shooting? Who pays for the ammo if you have to resit?