[ATD]

I am a new user of ANSYS AUTODYN, and I met obstacles during my modeling very quickly. I am modeling a pressure waves inside of a pyrotechnic actuator. At the beginning everything was pretty straightforward. I designed a 3D model in Explicit Dynamics module, then set up the AUTODYN model. However every simulation ended up with this same result. Namely the piston moved for some distance then stopped and went beck. After a while moved forward again. I have applied different meshes, different connections between material, etc. but no matter what I have done with the model there was this same result over and over again. I have no idea even when to look a problem. I will be very grateful for any hint. I set EOS of every part to „rigid”. Can it be the source of the problem?
Thank you