For such reason, those types of devices being affixed to the end of a barrel need to be pinned AND welded, deeming said device a permanent fixture on the barrel. If permanently attached, the length of said device increases the measured length of the barrel and therefore, the measured OAL of the firearm.

Under the old AWB, the permanent fixture (pin & weld) was the work around for post-ban threaded rifle barrels, especially if you wanted a suppressor.

With today's suppressor options, it's just an expensive commitment (i.e. Pinned & welded or integrally suppressed upper) to your build to have a suppressed barrel with total length of <17 inches and remain compliant in the face of the various AWB and SBR parameters (and potential ATF brace rules).