That may depend on where you live

There's overall length and then a permanent exenstion

“The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.”

https://johnpierceesq.com/when-does-the-length-of-my-flash-hider-or-suppressor-add-to-the-barrel-length-or-overall-length-of-my-firearm/#:~:text=In%20summary%2C%20the%20length%20of,the%20purview%20of%20the%20NFA.

Yep, as I read further in the link above it states that in a letter from the ATF.