Den Niro's character isn't the sort that would let his gun jam, you know he'd keep that shit clean and ready to go at a moment's notice.

Tbf jams just happen sometimes, it isn't really possible to eliminate it as a possibility. Though if you're really obsessive you can use a gun that is already less likely to jam than most and then work to get the spring pressure and everything just right to minimize the possibility as much as possible.

But yeah narratively, a gun jamming is pretty much the same as the guy slipping and falling over, or rocks falling on him, or God descending from the sky and taking his gun away. Unless it was already foreshadowed with neglect of the gun or some damage or whatever.