That was my original thought, but I had the gun checked out, cleaned and lubed about a year ago when my father gave it to his grandson. The gun has definitely seen a lot of use so maybe the firing pin or spring has finally given out.
I am still surprised that a light hammer strike could cause a hang fire. With other guns, I have had a few shells that did not detonate on the first hammer strike but did after the gun was re-cocked and the trigger pulled a second time. After this experience, I am going to take my time removing "dud" shells from the chamber.
