NONE - You are corrct about the inception of the Whisper concept - basically for urban "use"! But it has been taken a step further. The use of heavy projectiles with high ballistic coefficient (reduced trajectory drop - better energy retention); barely stabilised projectiles at subsonic velocities which tumble on contact (reduced carry in the event of ricochet and excellent terminal ballistics). I am not talking about using suppressors - apart from being difficult to engineer on a double - and illegal for some of us mere mortals in some places, the absence of supersonic blast just doesn't seem to scare the natives - whether two-legged or four. Taking the concept further for double rifles, reduced muzzle velocities in appropriate size cases has got to equal low pressures - which is what you always come up against in using shotgun actions to build double rifles. Who said you can't build a better mousetrap?