Question

What is the recommended method for attaching a haul line to a firearm? tie it to the sling. tie it to the barrel. tie it to the wrist of the stock. tie it to the trigger guard?

Answer 1

The answer would be tie it to the sling. 
This is to make sure it is empty and that you haul it up with the barrel pointing down. If by coincidental you let it down to quick and it smashes the ground check the barrel for mud or grass. Put a piece of black electric tape over the muzzle when hauling the rifle up or down from a standpoint.