The penalties for a NFA violation are severe and should be understood completely before you embark on ownership, work, transport, purchase or sale of a listed weapon. The federal government takes any violation, no matter how small, very seriously and the resulting fines, imprisonment and gun ownership ban can extremely harsh.
Make sure you fully understand your rights and limits when around a class 3 weapon or applying for a class 3 weapons license.
A violation of either gun act under the National Firearms Act can result in a felony conviction, punishable by up to 10 years in prison and a 250,000$ fine (26 USC sec. 5871). First offenses usually result in prison time, but the time spent can vary widely according to the specifics of the case.
The statute of limitations is 3 years for NFA violations, but can be extended to 6 years for some violations (26 USC 6531). The statute begins on the stoppage of possession, not the beginning of possession. If you illegally own a restricted firearm, you are in danger of being prosecuted.
In addition to any criminal proceedings, violators are also subject to civil forfeiture (26 USC sec. 5872). A forfeiture case is separate from any criminal proceedings and favorable outcomes in criminal cases do not release you from civil forfeitures (see US v. One Assortment of 89 Firearms, 465 US 354 (1984).
Use of a fully automatic class 3 weapon or a silencer in a crime can up the sentencing by 30 years even if no NFA prosecution takes place (18 USC sec. 924).
Even if you feel that the National Firearms Act is unconstitutional, ignore them at your own peril. The government and prosecutors take these crimes very seriously and you will pay a heavy price if they are ignored.