Is it possible to use the mouse as a substitute for the lightgun in any of the retroarch PS1 cores? If so, could someone please tell me how. Any help would be greatly appreciated.
Yes, but I don’t think it shows the reticle with beetle-psx-hw.
Try going to quick menu > controls > port 1 controls and set the device type to whatever the lightgun is (I forget what it’s called). If that doesn’t do it, try setting in port 2 instead.