Sawn-off shotguns are also illegal here in the Unites States, at least in Colorado if you just take a hacksaw to the barrel. A person can, however, have this done professionally. In this case it is legal, but the barrel cannot be shorter that 18.5 inches according to a local gunsmith. The main justification is home protection, but most people like to have assault shotguns so they can blow stuff up on public lands or other open spaces.