RubberEntropy, I don’t doubt your researches into the length of the belts - it shows a dedication to stamping out ignorance that is welcomed here at SDMB. 
However, there is that old phrase “correlation is not causation.” Even if the length of the belts of ammo is about 9 yards, you still have to show that the phrase actually dated to WWII, as others have commented. Anecdotal evidence, like “I heard it from a friend” won’t cut it. 
At any rate, welcome to the Straight Dope. 
Northern Piper, I make no claim to having found proof of any kind. In the absence of such proof however, the simple application of ‘Occams Razor’ must surely push this particular hypothesis a little further up the order of probability.
Thanks Rocket. We had a boy last week, Gabriel, brother to Josephine. Ten fingers and ten toes, all present and correct. Both kids are well, but Mum and Dad are looking like the living dead.
Hmm. I am still up for a day rooting around in the musty tomes of Parliament to see if we can find this phrase in print in Hansard. My first bet would be to concentrate on the period 1842 - 1849, based on your research in the thread started by RoseGryphn mentioned above. However, I shudder to think how much paper I’d have to cover unless we can track someone down who knows more about the specific period. I’ll contact some people who know about English history and see if we can’t get someone down there before too long.
Congrats to bwanasimba on Gabriel. My wife and I had our first (Kimberly) 11 months ago, and we haven’t caught up on our sleep yet.
Now back to the “nine yards”;
I feel I must point out that the basic theory behind the ammo belts being the origin of the phrase appears to me to be flawed. Once again, the phrase is understood to mean “everything”, or “the entire effort”. If you translate this into the context of airplane ammunition, it doesn’t seem to make sense. Here are some examples/scenarios to support my point.
Pilot returns to base and brags, “I gave them the whole nine yards”. He’s just stated that he used up all of his ammunition in engaging the enemy forces. What kind of warrior would he be if he didn’t use all of his ammunition? My point in this case is, he’s stating the obvious, and that doesn’t fit with the spirit of the phrase.
Pilot returns to base and states, “I gave him the whole nine yards”. What self-respecting pilot is going to admit to his peers that he used all his ammunition engaging one enemy target. In addition, with this scenario, it seems to me that the engagement would have to have been somewhat lengthy in the case if a dogfight, to use up all of the ammunition on a single foe. It’s my understanding that a pilot’s most vulnerable when their tailing an enemy plane, and the idea is to take a quick shot and break-off, to protect one’s own tail.
Or, the the case of a ground target, again the statement is redundant (hey guys, I just used up all my ammunition strafing that airfield!) DUH!
It seems more likely that pilots would be bragging about their minimal use of ammunition to destroy a target. Hence the phrase “killed two birds with one stone”, would be more appropriate.
I must comment on, “it seems to me that the engagement would have to have been somewhat lengthy in the case of a dogfight, to use up all of the ammunition on a single foe”.
“somewhat lengthy” is only about 30seconds of fire ! There are many accounts of prolonged combats where one or other adversary disengaged and fled due to lack of ammunition. The biography of ANY surviving WWII pilot, of any nationality, will yield such.
An outstanding WWII example would be the British ‘Short Sunderland’ flying boat which fought no less than six Junkers JU88’s in a combat lasting over an hour until the surviving Germans had used up all their ammunition and went home. Or in WWI, a Major Barker who, though badly wounded, fought 11 Fokker DVII’s to a standstill, winning the Victoria Cross for his efforts.
Many air combats that were not simply hit-and-run, involved a ‘turning fight’, where the adversaries flew an ever tightening circle attempting to get on the opponents tail. The advantage often shifed from one protagonist to the other, firing short bursts when the opportunity presented itself until one went down or broke off the combat.
I therefore believe a more likely emphasis, probably accompanied by the waving of arms in the mess, would have the expression meaning " I went all the way, fought him (them) to the bitter end, until all my ammo was gone, the whole nine yards, a TOTAL effort.
I was watching Squawk Box on CNBC this morning, and the host mentioned the phrase “the whole nine yards”. He hoped someone would e-mail him with the origin! 
RubberEntropy,
Thanks for the input on the length of time required to fire nine yards of ammunition. While 30 seconds of fire seems short, I submit that that is somewhat lengthy in the context of an air battle, considering the fire is typically in short bursts.
The point I was trying to make, and I believe you have reinforced is that the phrase “I gave them/him the whole nine yards” sort of loses it’s punch when you consider;
(a) Pilots regularly used up their ammunition in battle (hence stating the obvious);
and (b) if they used all of their ammunition (whether on (1)a single foe or (2)a series of targets), they’re either saying (1) I’m a pretty bad shot, or (2) I used all my ammunition (like I’m supposed to, again stating the obvious).
Pilots were not going to come back after a battle and say I saved some ammunition for my next time up. It seems to me they would have to be reloaded with a full “clip” every time they landed. If they had leftover unfired ammunition, it would have to be removed to load another “nine yards”, since that has been claimed as the capacity. Hence they would regularly use up their ammo, so that the ground crew couldn’t be overheard saying “that dumbass pilot, he left 3 and a half yards of ammunition in here”.
I will be swayed by documented references to the phase from WWII however. Here’s to discussion/debate.
Sheesh! I’ll be swayed by a quote from the Korean War! I’ll even be swayed with a quote from Viet Nam before 1967!
It occurs to me that the 30sec of fire may not be the significant number.
We have established,(see my earlier post), that a P51 pilot had only 22.5 seconds of fire from all six guns. Beyond that point he had a further 10 seconds of fire from 2 guns only.
Fighting with just 2 guns would put him at a distinct disadvantage. Perhaps the fact that these last 2 guns were the only ones with 9 yards of ammunition may explain the significance of the phrase to fellow pilots.
I must emphasise that I make no claim that the aviation explanation is definitive. I too will only be convinced by a contemporary printed reference. As to probability, the established facts concerning ammunition belt length do not disprove this explanation.