Solvent trap adapters are used to catch solvent, patches and residue generated from firearm cleaning, but they can also be used to make a suppressor which is illegal if not done properly. To legally convert a solvent trap adapter, users must file the proper forms and pay a tax.
Solvent trap adapters consist of a tube that can be screwed into a threaded gun barrel. The other end of the tube is threaded to fit a standard oil filter. This allows for all of the chemicals and waste to pass down the barrel and drain into the oil filter. The filter can be removed and the waste emptied when the cleaning process is finished. A solvent trap adapter kit typically has multiple adapter collars that allow the tube fit several different calibers of firearms.
The U.S. Bureau of Alcohol Tobacco and Firearms requires anyone that is attempting to build a suppressor file a Form 1 and pay a tax of $200, as of 2015. This requires the unit to be serialized with a traceable serial number. The ATF does not specify whether the collar of the tube constitutes the actual suppressor. The proper Form 1 filing does not allow people to sell any suppressors they make and license, only to possess them.