TyroPicard,
You never know, one of us might be right. Your story sounds at least as reasonable as mine, except I would have thought the Yanks might have been able to afford more than nine yards, especially with all that space available in the aircraft.
So, how many rounds in nine yards? I've no idea, and no intention of trying to find out. Anyway, if you knew that, and the rate of fire, you could work out how long the belt would last. I suspect not very long, and I have seen film of B-17 gunners ankle deep in empty cases. They obviously had a need to maintain fire for extended periods, not just ten seconds or so.
Also, how would the gunner know that there was only two yards of belt remaining?
However, I have been wrong once before.
Farmer.