There is no need to go to semi and fire off a shot before opening the receivers. The sector gear stops in the same position everytime, no matter if it's in full or semi. If it doesn't, then you have a problem with the PTW, and recent discoveries by users with this problem have identified it as a motor problem, or blocked sensor holes on the sector gear. Stoping in any position other than with with no more than 1-2 teeth showing on the sector gear at all is abnormal operation.
This is the maximum number of teeth that should ever be showing, and this does not engage the piston:
Change cylinders whenever you like, just be sure to follow the Systema recommended battery/cylinder matching. M90-M130 use 9.6v and M150 uses 12v only.