Probably not a good ratio
On the one paw, it neglects dropped bombs, artillery, and small arms. On the other paw, they use .50 in planes (including copters), tanks, and other carriers. Which spray down desert, buildings, rivers, whatever looks like it might be hostile. Or not as friendly as Detroit, say. A lot of the old ammo probably was quickly field tested, failed, losing a barrel, and pitched in the hopes that the bad guys would use it. Some of it might be dumped out of hand because they've lost too many barrels/receivers to verdigris covered rounds. I wouldn't use AK ammo from Vietnam days; WW2 is ridiculous.
How much of the dumped ammo is coming back to haunt the dumpers is up for speculation.
But you've got a point. We're killing too few bad guys for resourses spent (I consider our soldiers a lot more valuable than materiel.)
Perhaps if we knew who the bad guys were...