Regretably 'Madd Dogg's mansion' is a mission. Therefore the weapon 'works' because it is required to work. Many missions allow for special activity that will not work otherwise. (An example is the final escape from the Casino Heist where you can glide from the roof to another roof, but you can't do it otherwise.)
I've found that using the Desert Eagle instead of the Silenced Pistol is much more effective when leaving the MD mansion.