Those actions should be right
Compared to the "weapon\soviet\explosive_weapons\mortar\pm41_82mm_mortar_mp".
Set the following values:
weapon_bag\behaviour\
non_moving_setup: "true"
reset_rotation_on_teardown: "False"
I see that the weapon in anim_table also sets "60mm_Visible". I know that some british emplacements disable the main weapon while building, What unit did you use to start with? It might be related.