Somewhere in the back of my mind there is a vague song about this "9 Yards" but where and when I do not recall.

But a bit of Maths - US of Fghters WWII (Persuit =P) fired an 0.5 inch round, given the chamfer on the cartridge casing, and the joining links I would suspect One inch or possibly less per cartridege

Thus 1 yard = say 40 rounds.

9 yards = 360 rounds

Now a quickish Internet search Click 'Ere and scroll sugests that the P47 had an amunition capacity of 350 Pounds per gun.

Mathematics suggests that 8 x 360 = 2880 pounds or nearly 1.25 imperial tons. Hmmm ! I dont think so, this represents a substantial percentage of the aircraft weight.

Cut and paste from link -

empty weight            2,600 kilograms    5,730 pounds
   loaded weight          3,540 kilograms    7,800 pounds

Leaving no weight for Pilot, Gasoline, Oil, Oxygen etc


But 350 rounds suggests about 9 yards.

I do not claim that this origin of the phrase is correct. But it's room for thought.

Gareth