The usual suspects. [pdf]
2. Soviet Union
3. United States
4. Russian Federation
Number six in the sample is Romania, at half as much as Serbia. Some ammunition is manufactured in Russia but marketed by a US company. Others seems to be the result of captured US or US-supplied equipment in Iraq. Soviet is obviously quite old but then that country made enough bullets to fight a world war and supplied a bunch of proxy countries and other states over the years. What's remarkable though is the new manufacture stuff that's barely a year old.
You could say western action against ISIS is like cops taking the easy route and targetting the addicts and not the dealers.