Well, without more context, it's hard to give a detailed account. However, generally speaking, if it's a true story about an old man with a gun, it could involve the old man's life experiences. Maybe he was a veteran and the gun was a memento from his military days. And there could be a series of events in his life where the gun played some role, either in protecting his family or being part of a community story.
The 'an old man with a gun true story' could be really diverse. It could be that the old man had a gun because he was a hunter all his life and there are stories about his hunting adventures. He might have used the gun to fend off wild animals that threatened his property. Or it could be a story where the gun became a symbol of his authority in a small town in the past, and there are anecdotes about how he used that authority to resolve local disputes or protect the town from outlaws.
I'm not sure which specific 'an old man with a gun true story' you're referring to. There could be many such stories. It might be about an old man who uses a gun for self - defense in a dangerous neighborhood, or perhaps it's a story related to an old man who has a historical gun and the tales associated with it.