The WWII 8mm and .303 ammo that I've shot seems to have no difference in flash or smoke compared to modern surplus ammo and commercially avaliable hunting ammo, and a K-98 or Enfield barrel is shorter than the MG's they used in Spits, Hurricanes, and 109s.

Flash that the human eye can see in daylight conditions (even when looking at the muzzle of the weapon from the front) from a ~.30 cal MG are few and far between.