Env_joints have always been kinda flimsy, especially if they have to hull around heavy object with any speed. If you've ever played Gary's Mod and tried gluing a bunch of stuff together then moved it around, you'll know what I mean. When making weapons, less is generally better, since there are less things to hull around and less joints to be weird and stretchy. So making huge complicated things that all move can be difficult. Try messing with the mass of the env_objects or removing excess pieces and joints.
That being said things may have changed since I've done any modding and I'm not able to open it in TB atm. So take it with a grain of salt.