There are two things here that could help, but unfortunately, they are both have bugs associated with them at the moment that were recently reported (both within the past week and a half).
For the first one you would want to adjust the mass of the . Bug report ticket# is: UE-5642
For the second, you would want to use “Form Extended Structures” set to true under the “Flags” under the details panel in your DM. Bug report ticket # is: UE-6062.
With the second, this will work, to form extended structures BUT only one of the DMs will be destructible while attached, meaning you can shoot the other two but they will not destroy unless they become detached from the first mesh.