The way the law is written, the crime is the "intent" to silence a firearm (without prior govt authorization). It doesn't have to WORK, in order for you to be prosecuted and convicted!!!
There is one legal way to make a firearm "quieter", from the standpoint of the shooter, without going through the NFA process. However is cannot be used to any practical degree, on a handgun...make the barrel longer...
The further away from the ear the muzzle is, the quieter the sound of the shot. Of course, ultra long barrels have the disadvantage of cumbersome length, but they are quieter. There have been shotguns made with 40" barrels, for just that reason.