.60 cal

Here's something you don't see every day:  a manual for the .60 cal machine gun T17E3.

According to wikipedia, it was a reverse-engineered German MG151 autocannon, the primary armament in many German aircraft.  The Army was trying to find a use for 6 million rounds of .60 cal which had been manufactured for the recently obsoleted T1 anti-tank rifle.  Frigidaire produced 300 of the guns for evaluation, and there is some (weak) evidence that they were mounted on a P-38 for air-to-air testing.  The concept was ultimately abandoned in favor of the 20mm cannons which became the standard U.S. aircraft weapons after the war. 

While 6 million rounds might seem like a lot of ammo to abandon, during World War II even that amount was a drop in the bucket.  I'd be willing to bet that US industry was producing more than 6 million machine gun rounds every week during the war.  Hell, Ford's plant at Willow Run was turning out one complete B-24 Liberator bomber per hour during all of 1944. Yes, per HOUR. Do the math: that's over 700 planes per month. And that's just one model of plane produced in one plant. There were hundreds of plants turning out hundreds of different types of weapons at comparable rates throughout the war.  Ultimately, it didn't matter what particular weapons the United States deployed.  All that mattered was the fact that we could deploy more of them than every other nation combined.