The Front Sight protector is a Britishicon Service issue. It was used ONLY for Parade & Rifle drill use. It's purpose was to prevent the weapon snagging on any of the uniform at the Foresight protector. Or gas plug areas.
I have seen these advertised by a well known company for sale in the US. As, 'Brush Protector's'. This is TOTALY incorrect, the above statment is the correct one.
Common sense would dictate, that if you tried to aim the Rifle with the protector fitted, you would NOT be able to take a proper aim as the sight is then obscured!