It should not "slide off", all the washer, pin, and slot do is keep it from rotating. Inside the hider, a couple inches up, should be threads, matching the ones on the end of the bbl. The hider is screwed down tight, then backed off until the slot in the bbl aligns with the ears on the hider. The washer is dropped in place and the pin secures it. If your bbl is lacking threads and the hider slides on far enough to align with the slot, someone cut the threaded end off the bbl for some reason and the standard hider will not work at all. And JB Weld will not help. You have a few options:
Originally Posted by driswalds
1: High temp silver solder or weld the hider on.
2: Drill and crosspin the hider on, similar to how the sight base and gasblock are pinned onto an AK bbl.
3: Turn the area forward of the slot down on a lathe and sleeve over the bbl, leaving a thicker area up front to restore the bbl to original length and thread the end of the sleeve to duplicate the original setup.
The last option is the best, the other two may not even be viable unless the hider is a tight slip fit over the bbl.