The post ban Bullpup rifle barrel mounting system uses a barrel sleeve, split washer, locking nut and a jam nut. BATF approved the design on the threaded muzzle during the Ban as there are not enough threads to support a flash hider. If a flash hider were installed it would come off under fire.
The jam nut and the locking nut are the same part but have different functions. When the barrel is mounted in the receiver the barrel sleeve slides over the barrel so there is pressure between the locking nut and the receiver end cap. The sleeve is slid over the barrel then the split washer is placed on the muzzle and the locking nut is turned on until it collapses the split washer and is snug against it. A drop of # 271 Loctite is used on the locking nut. If the locking nut is tightened too much it can bend the barrel. The jam nut is then installed to keep the locking nut in the proper position. When the rifle is fired the barrel heats up so it will expand and the split washer will open up. When it cools the split washer collapses again.
On the pre ban rifles the barrel had longer threads on the muzzle so the flash hider was used as the jam nut against the locking nut & washer. If the locking nut is not used the flash hider could become loose. An A1 flash hider was used because it did not need to be indexed.
To install a flash hider on the post ban rifles the barrel sleeve must be shortened to expose more threads to support the flash hider. The barrel sleeve must be shortened from 3.6" to 3.445". A new shortened barrel sleeve is available. It is part number 30-202L-3. The locking nuts have # 271 Loctite on them so they should be heated with a propane torch to soften the Loctite for easier removal with a spanner wrench. We use the FN gas regulator tool part number MMT-0003 for. With the sleeve removed you can place it on the outside of the barrel against the end cap of the receiver. Mark and cut the sleeve so that it is just ahead of the shoulder of the barrel threads.
The barrel sleeve split washer and locking nut should be installed as noted above. A flash suppressor can then be installed as a jam nut. It is best to remove or install the locking nut with a spanner wrench. Use the spanner wrench to hold the locking nut in place when tightening the flash suppressor against it so that it does not move. The Loctite can be used when installing the flash suppressor as well. An A1 flash suppressor, Izzy brake or a Phantom flash suppressor is recommended as they do not need to be indexed. Adding a peel washer or crush washer to index a flash suppressor takes up threads that are needed to support the flash suppressor. If there are not enough threads to support the flash suppressor it can come off under fire.