the choice of powder is very important. Nowadays modern armies tend to use smokeless powder and in general using loadings that would avoid flames, which can still occur in cases like long bursts, when partially combusted powder gets pushed out of the barrel and ignites as the next round comes out. Sometimes light conditions, moisture in the air, the moon phase as wisely quoted by our esteemed colleague GnigruH (LOL!) and other amenities can affect the visibility of the flame.. rest assured that there's a flame coming out of there anyway, if you dont believe me put your hand close to a muzzle of a rifle when is firing and get ready to smell burnt chicken