If you’re at all into the Halo modding scene, you'll have likely seen RejectedShotgun on YouTube. They're well known for pulling off what I can only describe as feats of incredible ingenuity, creating mods that many, myself included, thought were impossible.
Before I dive into the details, it's important to mention that this is for Halo 3 on Halo: The Master Chief Collection, and the mod is accessible via Steam Workshop and isn't available on console.