To obtain an item regulated under the National Firearms Act (NFA), like a suppressor (often referred to as a "silencer"), short-barreled rifle (SBR), short-barreled shotgun (SBS), or machine gun, you generally need to complete ATF Form 4 and pay the associated $200 tax stamp.
This tax stamp, administered by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) under the National Firearms Act of 1934, essentially serves as a federal tax levied upon the transfer or making of such items.