This is just one of those things I've learned to live with, but I was wondering if there was a reason why it is possible to have two parts sharing the same space in a Motion study using a contact in which the default material is left at Acrylic (see attachment). If enough impact velocity exists, I have had the parts go through one another.  I usually then change this to Dry Steel.  Turning off the Material selection also produces the correct behavior.  Nylon seems to be somewhere in between.  This is supposed to be a rigid body analysis, an attribute of which is that the deformation of the material is ignored.  So, what gives?   (pun intended).