A silicone gun is too hard to control using a computer, so I removed the handle and put on an extruder drive.  I used a tiny 100RPM 12V DC motor and it seems to have enough torque! More testing to come.

And the 3D printed version looks like:

And the motor (yes, it is a 6mm threaded rod):

For more details, have a look at https://cad.onshape.com/documents/bdf821161c0d3c01945c5e17/w/145cfc7caf50a052aa41e42f/e/ac6f177acba94f862104b01a