This paper model is a Lassiter, a valuable, antique laser pistol which was the forerunner of modern laser technology, based on the TV series Firefly, the papercraft created by RocketmanTan. It was in the collection of Durran Haymer when Saffron joined Malcolm Reynolds to carry out her plan to steal it. According to her, it was one of only two known to exist. The theft was successful, ultimately leaving the Lassiter in the hands of Mal, despite Saffron's attempts to take it for herself, although he later mentioned that he was having trouble fencing it due to its great value. Inara comments that though it's no longer functional, the Lassiter is still worth a fortune.
You can download the paper craft model here: Firefly - Lassiter Pistol Free Paper Model Download