Just making it clear that airsoft silencers do indeed work and that I doubt the authorities would care whether or not your nerf gun has a silencer. And plus, what kind of ranges are you shooting from? I mean, it's not going to make that big of a difference whether or not it's silenced.

Umm, maybe off topic now, but I just had to say this. Airsoft silencers do in fact work and are not " just a piece of PVC fitted over the barrel". Many are made of metal and have foam in them. I have seen, felt, and used many silencers and can say they all work to a certain degree. I will have to say that most of the noise comes from the gearbox of an AEG (Automatic Electric Gun), but the noise that comes from the air released into the barrel and out at the muzzle is minimal to none with a silencer on.

Honestly, I doubt anyone will care if you have a silencer on your nerf gun. Many airsofters (myselft inluded) have "silencers". They look like silencers, but do very little, if anything, in silencing.

As stated by the ATF (Bureau of Alcohol, Tobacco, and Firearms):
(A9) Q. Are Paintball and/or Airgun Sound Suppressers legal? [Back]

A. §921(a)(24) The terms "firearm silencer" and "firearm muffler" mean any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for use in such assembly or fabrication.

Numerous paintball silencers tested by the Firearms Technology Branch have been determined to be, by nature of their design and function, firearm silencers as defined in 18 U.S.C., Section 921(a)(24). An individual wishing to manufacturer a firearm silencer must receive prior approval from ATF by submitting an ATF Form 1 and paying a $200 making tax.

Now all you have to do is figure out whether or not a Nerf gun is classified as a firearm.