It's possible that there's a hidden stash of Muggle items in Hogwarts, and Harry stumbles upon it. He discovers the gun there, realizes it could be a powerful weapon against Voldemort, and decides to use it.
He could have been influenced by the Muggle world. Having grown up with Muggles, he might have seen the power of guns and thought it could be a more straightforward solution compared to complex magic spells. Also, he might have wanted to prove that Muggle technology could be just as effective in the wizarding world's battles.
The wizarding world would likely be in a state of shock and then celebration. People would start to see Azkaban in a new light, perhaps no longer as just a place of horror but also as the site where the great evil was finally vanquished.
Some might be shocked. Hermione, for example, would probably be very concerned about the use of a Muggle weapon in the wizarding world and its implications. She might worry about how it could change the balance between the Muggle and wizarding communities.
Yes, there might be such fanfictions out there. Fanfiction writers often explore alternative storylines, and the idea of Pettigrew turning on Voldemort and killing him is an interesting and unexpected twist. It could involve Pettigrew having a change of heart due to some complex reasons, like realizing the true evil of Voldemort's ways or having a personal vendetta against him that overrides his loyalty.
The wizarding community would be shocked. Guns are not part of their world, and for Harry to use one to kill Voldemort would be unthinkable for many. There would be outrage from some traditional wizards who believe in magic as the only proper way to fight.
Well, it would change the whole wizarding world's concept of combat. Guns are a Muggle weapon, so it would show that Muggle technology could be a real threat to wizards. Maybe it would make the wizarding community more open to learning about Muggle ways.
Well, in some fanfictions, Harry might use some hidden magic he discovers in Azkaban. Maybe he finds a way to turn the Dementors' power against Voldemort. Since Dementors can suck out a person's soul, Harry could somehow direct them towards Voldemort, weakening him enough to deliver the final blow.
Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
Well, it completely changes the nature of the magic - based world. Usually, the battle between Harry and Voldemort is about magic spells and wizardry. With a gun, it brings in a Muggle weapon, which might make the wizarding world seem less special. It could also change the way other characters view Harry. Some might see him as a radical for using such an un - wizard - like method.