A while ago Sig Sauer introduced a new line of pistol-caliber carbines, including a model with a built-in "muzzle break" called the MPX-C.

The MPX-C has a 6.5" barrel with 9.5" muzzle brake permanently attached. Basically it's a monocore baffle stack permanently attached to the end of the barrel. You buy the "silencer" separately, which is simply a hollow tube that screws over the baffle stack.

This is in line with many previous ATF classifications that consider the "serialized part" the actual silencer for NFA purposes. Many suppressors are now user-serviceable and the "serialized part" is just the outer tube.

Sig has sued the ATF for classifying their muzzle brake as a suppressor, claiming economic injury.

Another interesting case to watch!

Read more here