So, Harry is carrying around a Chekhov's Gun in the form of an actual gun. Which raises a few questions, such as:
- What can it actually do? He thinks it's conjuring new bullets, but he hasn't had a chance to test its limits. Does it have a finite total amount, or a limited rate, or a risk of overloading?
- What kind of targets will it be useful for? Maybe it would be effective in the final post-Horcrux confrontation with Moldyshorts? Or will Harry take a darker path and assassinate some of the Death Eaters? Or perhaps he'll encounter magical critters that need a good shooting, though he wouldn't be able to use it in the Triwizard Tournament without exposing himself.
- More immediately, how did Mr Weasley not notice it was missing? When Harry first discovered it, Mr Weasley called it "one of our more interesting finds", described it in detail, and said that everything on the shelf would be destroyed because "we can't let it fall into the wrong hands." It's rather odd that he'd apparently just forget about the gun when the cleanup came around. If he knew about the time travel at that point, I'd say that maybe he suspected Harry but trusted him to know what he was doing; however, he didn't find out until a year later.