With Bungie you never know if it's actually a key to a Easter egg.
The Stranger hands it to you. This could be Bungie's way of saying this is important for other things.
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Dismantled that shit ages ago for vault space. Why would I keep some shitty rare? A question I no regret answering by holding down the square button. They better not make the Stranger's Rifle a requirement for No Time to Explain. They should KNOW people deleted it since they can basically see everything that everyone is doing.