Create a "trigger" using a small paper lever or a folded hook.
Roll a short, wider tube and mount it on top of the barrel using tape or glue for that "assassin" look. Pro Tips for the "Assassin" Aesthetic Sniper+assassin+hack
Roll multiple sheets of paper tightly into long tubes. Slide one tube inside another for extra length and reinforcement. Create a "trigger" using a small paper lever
If you're a fan of games like Sniper 3D Assassin , you can model your paper gun after specific in-game rifles like the Kar98k or AWM by adding a "bolt-action" paper handle. Sniper+assassin+hack