I always tie-off to my sling as well as some of the other posters. Again, key is to make sure that there isn't a bullet chambered when ascending and descending.

You may opt to use a "finger glove" and place it over the barrel to ensure that no debris gets in. ("Finger glove"--not sure of the correct terminology--is similar to the finger portion of a rubber glove and resembles a small rubber--sorry for the comparison.)