They are grouped together so they are considered 1 object. The trigger is surrounding the weapon and affects the area around it. Its how on surf_buck-wild the made the usp have a car attached and the shotties have corn, a buck, and smoke. Some-what simple technique. You should try it.
*NOTE* The FPSBanana Forums did not make it easy to create this diagram. Its the best i could do. Hope it helps.