In good writing, details serve multiple purposes. Just showing a gun because it will be used later will seem ham-fisted to an audience. A good writer will introduce the gun as a character moment and as setup.
Maybe the rifle/gun is just a bad example today? I watch movies all the time where there are guns or rifles which does not gow off. And I sure don't feel like some promise was broken.
I think it’s something which is more obvious in a literary context where not talking about the gun means there is no obvious reference to it, but in a movie it may still exist, they just do not draw attention to it.
Gun isn’t literal here. Gun is a metaphor for details; if the details don’t serve the plot (if the gun doesn’t go off) then the details should be removed (don’t show the gun)
E.g. that the character is the kind of person who owns guns.
It does not go off or get used, but the viewer will use that as input to make a judgement about the character.