It is horrible, seizure-triggering flash in RGB or thermal mode. But thermal mode is like camera flash in my eyes constantly due to contrast.
Note: I think it won’t be fixed anytime soon, since it is intentional;
From some patch notes: “Now guns of all calibres, from machine guns and autocannons to tank and naval guns, will receive the new shot effects. Muzzle fire is now modeled with particles, it has reliable reflections on the surrounding surfaces and perspective correction. The size of the flash depends on the calibre of the gun. The flash from a shot has received the property of photometric brightness, making it look more realistic.”
Also: (all) 40mm muzzle flash is WAY bigger than it should be (Update) // Gaijin.net // Issues