Note that in this time frame the United States manufactured 2,879,148,000 rounds of rifle and machine gun ammunition.
The image comes from a rather lengthy article on World War I statistics, which you can read here: http://net.lib.byu.edu/estu/wwi/memoir/docs/statistics/stats8on.htm Among the many interesting statistics, diagrams, and tables in the article, there is this interesting quote regarding small arms ammunition:
Small-Arms Ammunition
A-sufficient supply of small-arms ammunition has always been available to provide for troops in service. The complication due to the use of machine guns and automatic rifles of French caliber has been successfully met. To meet the special needs of the Air Service and of antiaircraft defense, new types of ammunition have been designed and produced, the purposes of which are indicated by their names---armor piercing, tracer, and incendiary. Before the end of the war American production of small arms ammunition amounted to approximately 3,500,00O,000 rounds, of which 1,800,000,000 were shipped overseas. In addition, 200,000,000 rounds were secured from the French and British.
In reading on WWI ammunition production, I also came across an interesting article on the "Doughboy's Rifle." I can't vouch for its veracity without doing more research than I have time for, but you're welcome to read up on it. The link is: http://www.worldwar1.com/dbc/dbrifle.htm