You linked to Snopes twice. I read Snopes and found it inconclusive.
You're saying GOD, WHY DID YOU SEE THIS STILL-NOT-ACCURATE IFORMATION IN A WIKI NOTATION?! Simmer down. ;P
'Still not accurate' because no actual research beyond the couple examples has been done. Yes, 27ft of side-by-side .50 rounds would be, well, a little more than the number of inches (324 inches. A .50 case is about .9" at the base). Call it 350. We'll assume the cases touch at the end...seems like I've seen old cloth belts where they don't, but I'm assuming all the planes had metal disintegrating links.
There were planes with .50's that held anywhere from ~200 rounds to 4-500 rounds per gun. This could also refer to bomber turrets, which IIRC on a B-17 or some such could hold 500+.
The important thing to note is the inconsistency. Which leads me to believe that someone either had, or thought they had, nine yards of ammunition, and the phrase was popularized. It's not like the military was issuing belt of ammo in crates with 'BALL AMMO .50 CALIBER 27 FEET' stamped on the side. However, since the ammo was surely shipped in belts that were not the total capacity for a gun, one would wonder how long they were before being chained together and/or truncated to fit a plane's ammo capacity.
Also, I would bet 20mm belts were more likely to always come in the same size...nine yards seems like 150-200 rounds, IIRC the typical capacities for those guns.