mhh, maybe if you play with the values for the morning star in the tables you can get such weapon... anyone can tell more about this?
Theoretically, if you made a weapon with a stupidly high mass number you could do it. Unfortunately it seems to misbehave if you set weapon mass high enough... It stops pushing and just rotates the target really really fast. I just tested it. I would have loved to make a hacked weapon that'd send the Lucy careening away at 2000 meters per second... *sigh*
It makes sense if you think about it.
What happens when you have a bottle standing on a table, and you apply force on it's lower part? The result varies depending on the force. If you push gently enough, the bottle slides on the table. If you whack it with your hand, you'll see the bottle pivoting around it's center of gravity rather than moving forward.
In other words; when you hit something so that the impulse (or force) vector is not aligned with the center of gravity, it causes a momentum, which tries to make the thing you hit rotate.
You would have to hit targets "dead center" with a stupidly high-massed gun to make them primarily move backwards rather than rotating. Undoubtedly, the physics is messed up in FS2 too, but there is a modicum of realism in this behaviour... to some extent anyway.