Buy of it never failing. When ever you pull the trigger a round will go off. When you pump it, a round will always come out, it will grab another out of the tubular magazine and put it back into the chamber for you to pull the trigger again.
I carried one, for over a year in afghanisthan. It never once failed. I honestly only cleaned it very lightly and a drop of oil when cleaned, that whole time. I own one my self. Between the military and my personal one, i have tens of thousands round. They have always fired when i wanted them to or needed. These are extremley reliable weapons.