NamcoGunCon: Use correct GPU clock as reference

src/core/gpu.cpp

@@ -393,6 +393,11 @@ void GPU::DMAWrite(const u32* words, u32 word_count)
  * PAL - sysclk * 709379 / 451584
  */
 
+TickCount GPU::GetCRTCFrequency() const
+{
+  return m_console_is_pal ? 53203425 : 53693175;
+}
+
 TickCount GPU::CRTCTicksToSystemTicks(TickCount gpu_ticks, TickCount fractional_ticks) const
 {
   // convert to master clock, rounding up as we want to overshoot not undershoot

src/core/gpu.h

@@ -173,6 +173,9 @@ public:
   // Converts window coordinates into horizontal ticks and scanlines. Returns false if out of range. Used for lightguns.
   bool ConvertScreenCoordinatesToBeamTicksAndLines(s32 window_x, s32 window_y, u32* out_tick, u32* out_line) const;
 
+  // Returns the video clock frequency.
+  TickCount GetCRTCFrequency() const;
+
 protected:
   TickCount CRTCTicksToSystemTicks(TickCount crtc_ticks, TickCount fractional_ticks) const;
   TickCount SystemTicksToCRTCTicks(TickCount sysclk_ticks, TickCount* fractional_ticks) const;

src/core/namco_guncon.cpp

@@ -169,7 +169,8 @@ void NamcoGunCon::UpdatePosition()
   }
 
   // 8MHz units for X = 44100*768*11/7 = 53222400 / 8000000 = 6.6528
-  m_position_x = static_cast<u16>(static_cast<float>(tick) * (1.0f / 6.6528f));
+  const double divider = static_cast<double>(m_system->GetGPU()->GetCRTCFrequency()) / 8000000.0;
+  m_position_x = static_cast<u16>(static_cast<float>(tick) / static_cast<float>(divider));
   m_position_y = static_cast<u16>(line);
   Log_DebugPrintf("Lightgun window coordinates %d,%d -> tick %u line %u 8mhz ticks %u", mouse_x, mouse_y, tick, line,
                   m_position_x);