The way you bulk fill a pistol is to cock the gun to open the valve. Connect a bulk fill setup on the gun and dump CO2 into the pistol to chill it and dump the CO2. Close the valve and open the bulk again to fill the gun. To do it properly the gun should be weighed as it is filled to determine the correct amount of CO2. If you over fill the gun it will freeze up and you will get valve lock.
Many guys just guess at the amount and that will work of course sooner or later you will screw up…LOL