How about selectable sub-sonic/normal ammo on the usual auto/single fire mode key ? The normal ammo wears out the silencer quicker but has increased range (should it be a "suppressor", or maybe they were called silencers in the 40's, anyway I'm being very pedantic). When the silencer wears out using normal ammo the weapon makes a lot of noise and you have to pick up a new one for a new silencer.