My favorites are silenced sniper rifles- so you silence the boom from the gasses being ejected out of the barrrel, but the bullet is still moving at supersonic velocities still giving off a loud 'CRACK' sound. Besides- the bullet would hit the badguy/goodguy before they could possibly hear the rifle going off anyhow. If you don't believe me that the sonic boom from a rifle is very loud, work the pits in a high power rifle event- you need to wear ear protection not from the report of the rifles that a several hundred yards away, but from the sonic boom crack of the bullets flying overhead.

The closest thing I've fired to silenced firearms was CB caps out of a .22 rifle- quieter than an air rifle.