[MegaDeKay]

Sinden is no longer the way to go. Most lightgun enthusiasts have now gone the Gun4IR route [0]. It uses the IR sensor from a WiiMote plus a microcontroller in the gun (either a gutted commercial controller like the PS Guncon, a modified Nerf or similar, or something straight up 3d printed) and four IR LEDs placed around a monitor / TV at the midpoints of each each. This system is extremely accurate and there is no flashing border around the screen like with Sinden. Unfortunately, the whole shooting match (see what I did there?) is closed source code and (as of now) Window's only for the calibration-based PC software.

The current open source competitor to Gun4IR is the Samco light gun [1]. It uses four LEDs as well, but with two on the top edge and two on the bottom edge of the screen. A couple Wii LED bars will do the job here as well. I don't think it is quite as accurate as the Gun4IR as I don't think it accounts for perspective correction if you move from the position it was originally calibrated at. But...

Sam & a few others are readying a new design called OpenFire [2] that will be at least on par accuracy-wise as Gun4IR and will be fully open source and cross platform. It should be available relatively soon. Pair this with the PiCon [3] and you have a lightgun with a pretty crazy feature set. All the guns mentioned support some kind of solenoid & rumble support, but the PiCon kicks it up a notch with exclusive OpenFire features like an OLED display, NeoPixel LED, accelerometer, and analog joystick.

[0] https://www.gun4ir.com/

[1] http://forum.arcadecontrols.com/index.php?topic=160517.0

[2] https://www.youtube.com/watch?v=aE9a-fsnMwU

[3] https://diylightgun.com/lightgun-details/?lgid=506

edit: make more specific reference to OpenFire