Re: \"Super-sonic\" question...
I believe you'll be happier calling them "suppressors" rather than "silencers".
They suppress the noise associated with the expanding gases used to push the bullet from the bore. They DO NOT suppress the sounds made by the projectile.
The super-sonic "crack" is a projectile related sound/event. The projectile is beyond the confines of the barrel and suppressor when the "crack" sound is made so there's no way for the suppressor to do anything for this type sound.
Suppressor "suppress" the muzzle blast sound signature, muzzle flash, dust/debris signature and reduce recoil. They also get HOT and create mirage problems when used in sustained fire situations.
If you fire sub-sonic ammunition and have a quality suppressor the critter you are shooting (or shooting at) will be able to hear the round coming (and if you miss) will also be able to hear the round going past and beyond. This is because the projectile is traveling slower than the speed of sound allowing the "whoosh" sound of the disturbed air to arrive before the projectile. After a little practice people can often easily identify the direction of the source of the incoming projectiles when sub-sonic ammunition is used. When the rounds are super-sonic the "crack" is made by the pressure wave pushed by the projectile will be heard as if it came from about 90 degrees to the source of the projectile. The target will not be able to hear the round come or go, it will be able to hear the secondary "cracks" as the projectile passed obstructions along it's flight path and the impact sound. The projectile impact sound at it hits the intended target or anything else is of varying intensity and can be quite loud...we often hear this same sound when shooting once the distance (time of flight) is sufficient for the two sound events to be differentiated by our ears.
They're suppressors, not silencers.