ORIGINAL: RCGuy41
G-Pete,
It's a Flash surpressor for either a machine gun or cannon. Primarily used on night fighters, RAF Lanc gunners had them on their browning 303 machine guns to prevent blindness when firing the guns.
Alright Bradley - you are the first with the right answer - this one fits 303, .30 and the .50 cal. The 20mm had larger ones.
We looking forward to your question...