Any way to count them?
I think that you would bennifit from looking up the history of the M-16. Part of why it was designed, was that troops in WW2 would return with either the load out they were issued or just slightly fewer rounds.
So that sounds like someone was keeping data on rounds expended in WW2 on the American side. For the rest of the combatants, thats gonna be harder.
Declan
Not quite what you’re looking for but here’s a related statistic that’ll give you some idea of the scale you’re looking for: the United States Army Air Force reported that it dropped 7,952,020 high explosive bombs in combat during World War II.
A cite in Barbara Tuchman’s *The Guns of August *(about World War I) talks about a shipment of 40,000 rifles and one million rounds of ammunition – presumably 25 rounds for each gun in the initial shipment.
In a report by Colonel Hays Park to the USMC, he quotes statistics from the US military records regarding the consumption of ammunition in recent wars.
“In World War II, the United States and its allies expended 25,000 rounds of ammunition to kill a single enemy soldier. In the Korean War, the ammunition expenditure had increased four fold to 100,00 rounds per soldier. In the Vietnam War, that figure had doubled to 200,000 rounds of ammunition for the death of a single enemy soldier.”
The above was extracted from the book: “Snipers” by Craig Cabell and Richard Brown.
John Blake Publishing;
London 2005.
ISBN 1 84454 131 2
The thing is, ammunition keeps for rather a long time, so there’s no point trying to work out how much was produced, because lots of it is still being used today by military rifle shooters, 65 years after WWII ended. And when WWII kicked off, you can bet soldiers were being issued ammunition left over from World War I.
So, in short, I don’t think it’s actually possible to definitely answer the OP’s question with anything more specific than vague educatd guesses or an unhelpful “Lots. Lots, and lots, and lots.”
Peloponnesian War-zero. 
Wouldn’t it be as simple as calculating “ammo in stock at war’s outbreak” plus “ammo ordered and received” minus “ammo in stock at war’s end”?
Do you count target practice/training as “bullets fired in war”?
Aww, I was going to make the same joke about the Punic wars.