not so - it has to do with length of cloth on a bolt.
It is too so. The length of a .50 ammo belt for American fighters in WWII was 27 feet, or 9 yards. This wasn't just the spec for Mustangs, but also Thunderbolts, Corsairs, etc. The length was standardized and fighters were designed to take this length of ammo belt. Giving somebody "the whole nine yards" meant lining them up in your sights and pulling the trigger until the guns all went "click."