k31 threaded barrel

are all k31 barrels threaded or do i have something rare?

Nope. None are threaded and your's isn't rare. That was doen with one of our threading kits and you also have one of our muzzle protectors. Its done that way to accept both our Muzzle Dampers and/or Threaded Muzzle Brakes.
All the photos are here.
Swiss Products

Latigo

thanks for the info