The Separation/No Sliding contact allows contact faces to separate, but prohibits relative sliding when they touch.
For the Actuator-Plunger interface, the Separation/No Sliding contact is appropriate. When the trigger is pulled, the actuator is pushed forward. This results in separation between the top surface of the plunger and the actuator. At the same time, engagement occurs between the bottom surface of the plunger and the actuator. It is reasonable to assume that the engagement/separation occurs without slippage between the actuator and plunger.
In the following image, note that the surfaces of the plunger and actuator are split into multiple faces. In this manner, the contact surfaces are more explicitly defined.