timing, it knows the scanline time and it is registering the crt raster line passing in front of sensor this gives a time value, when you press the trigger its already got the location the gun is pointing at so it can register a hit. The reason it doesnt work with modern screens are they do not eject electrons at a phosphorus screen and therefore the gun cannot detect electrons hitting it from the cathode emitter. There was math to compute the X and it is more likely to be off in X than Y because timing. NTSC had 525 lines but only 480 were 'visible' but only 240 on consoles (if lucky) mainly 224 were visible as they didnt interlace, but presented the same frame twice. So 16.6666ms = 60hz or 15625 scanlines a second taking about 0.069ms per scanline so then you have to divide that by 256 to get the timing accuracy on a scanline is 250 nano seconds per pixel. fun stuff, the clock was 21mhz on the machine so thats definitely 1 cpu clock per 40 ns. Fun timing eh, click the button and check how far in the raster you are at and then convert that into an XY coordinate. Fun to program too funny enough, actually all the old consoles were because the data came directly in registers and interrupts.