Stanwell,
Got out the magnifying glass (old eyes not so good today). Clearly a thin metal hollow tube in the right place.
You are correct, Sir ! - thank you.
(Used on occasion to make unwelcome appearance on firing) - the gun mountings were not man enough for the job, vibration worked the gun loose. First sign was when the rounds started to touch the tube side on the way out. They imparted enough push to slide the tube slowly forward and out (it did not seem they were locked in position in any way). If you kept on firing (not recommended), or had a "runaway gun", things got worse and the rounds popped out all over the leading edge.
A rare occurance, as you were lucky if the (original fit) 0.300 Brownings got 20 rounds away before a stoppage. That was one reason why all the Mks I and II supplied to you and to us in India had the rear two replaced with British 0.303s. Also, the 0.303 was a "open-bolt" gun, so could not (should not ?) runaway unless the trigger was pressed or defective. We did not bother with the front ones, as they were hardly ever used.
Don't know anything about the 0.50s in the Mk.IV.
Danny.