It's perfectly legal to buy, make or posses an airgun silencer. It must be made specifically to fit an airgun and only intended for that purpose.

According to the ATF, any airgun silencer that's removable, not an integral part of an airgun, or is removed from one in those two categories, could be considered an illegal silencer. Intending, attempting or actually mounting one on a firearm is illegal. If you're not planning to use one on a firearm, you're not breaking the law.

To put this in a clearer context, if you're in possession of an automotive oil filter, you could be arrested and prosecuted for possession of an illegal silencer. There would have to be evidence you intended to attach it to a firearm. Oil filters are widely known to make very effective silencers. Any object that can reduce the report of a firearm could be considered illegal if used for that purpose. You aren't going to be arrested or charged with a crime for simply possessing an airgun silencer. There has to be reasonable evidence you mounted it on a firearm or intended to do so.

There was at least one case where this happened, it was unfortunately used to create hysteria over this subject in the airgun community. In that case the offender was under investigation for serious criminal activity and was a previously convicted felon. He ordered a big-bore air rifle with a silencer and was illegally in possession of a firearm and explosives. The airgun silencer was alleged to be intended for use on the firearm. The charge for that was thrown out in court because that intent couldn't be proven. He was sentenced to a lengthy jail sentence for the various felonies he committed, not for the airgun silencer as many were lead to believe.

If you're not planning to mount an airgun silencer on a firearm then you''re not breaking the law. Just don't do something stupid like carry or store a firearm with an airgun silencer or oil filter. It might also be a bad idea to buy an airgun silencer if you don't own an airgun but do own firearms. These actions could look suspicious to law enforcement. If you're not involved in criminal activity in the first place it's unlikely anyone would notice or care.