I would probably investigate a Serial-to-USB adaptor for your device. You are far more likely to find a way of interfacing with it as a USB device rather than a Serial device.
Its also likely you may need some sort of Serial->USB software/driver installed on your PC to facilitate this.
Not sure if that is of any help at all, but its certainly what I would start looking into if it was me.
I had an old GunCon gun from a Playstation that I connected to my Arcade Machine using a serial adaptor and a ropey bit of software that gave me rudimentary support for the lightgun for my gun games... but it was really unstable. In the end I ditched it all, but im pretty sure a much more expensive USB lightgun would have worked perfectly.