on Atlas
have turret equiped but when i go to group screen not only does the equipped turret disapper that actual slot disappears also - slot does not reappear unless i do "unequip all"

don't think turrets are automatic - - - but what do i know

so how do i assign turret to a mouse button?

You can't, turrets can only be used by other players. I think the /gunner command can tell you more.

if you are in a turret then it uses the +shoot3 command iirc