Ok i looked at the example map i made with the tire swing which I think is basicly the same thing your doing but with a barrel instead. You use a move_rope entity and place that where your rope starts ( what it will be attched to, like a branch of a tree for example on a tire swing) then you make a keyframe_rope and name it something like keyframe_rope1, and place it where it conects to the barrel. Now name the barrel to something like swinging_barrel or whatever and amke sure the barrel is a prop_physics. Go into the keyframe_rope entity and in the parent field put the name of the barrel, in this case "swinging_barrel". Now go into your move_rope entity and put in it the next keyframe field the name of the keyframe_rope entity, in this case "keyframe_rope1". Now make a phys_lengthconstraint and put it in the exact same place as the keyframe_rope entity and name it something like "lengthconstraint_1". Now go to the "entity 1" field and type in or select the name of the barrel, which in this lil guide would be called "swinging_barrel". Thats it, tell me how it goes!