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The Snape Viper was a repeating crossbow designed by Tobias Luca Snape and Eileen Valora Prince. Tobias had the main plans for it saved for years in his vault in Gringotts, and finally delivered a group of them along with the plans to Hogwarts through the floo system.
"In case you're in a place where wands won't work, you'll need these."- TobiasThe crossbow was an extremely swift weapon, using its own recoil to cock itself.