An article was posted here stating that in Iraq the ammo consumption peaked at 70 million rounds per year.
These purchases are 10 times that level.
Your theory doesn't fit these numbers.
The US Military used around 250 - 400 million rounds per year for Peace Time training and limited peace keeping operations in the early 90’s up to 9/11
Yearly US military demand for Ammo since 9/11 has been over 1 Billion rounds per year for every year and is was generally around 1.25 - 1.5 Billion rounds per year during the peak of the conflict.
There has been a serious ammo shortage since around 2004 and ammo prices had doubled or tripled.
It is just now that ammo is becoming freely available and we are also seeing some price reductions as well.
Below is an article on ammo usage and supply from 2009 that covers the topic pretty well.
Fun Fact on Ammo use.
Our military averaged 250,000 expended for each terrorist they offed.
This is up from the 30,000 round per VC casualty from the Vietnam conflict.
The 250,000 round number is a statistical average that smears all ammo use over the total number of enemy KIA. However, the political pressure to limit the number of collateral civilian deaths meant that a disproportionate number of enemy KIA in the recent conflict were taken down by Snipers or Marine and Army Designated Marksmen.
These kills were usually made using one shot per kill and almost always less than 10 shots per kill. Our Snipers and Marksmen are darn good and recent advancements in optical sights made them even better.
Taking this into account, the real number is probably more like 500,000 rounds per kill or higher for many non Sniper, non DM engagements. That's a whole lot of rounds expended shooting (or missing depending on how you choose to look at things) to take down each individual terrorist.