Many years ago we used to have an annual allotment of ammo for training. If any was turned back in then your allotment for the following year was reduced. I spent several days on the range a couple of times a year with the other gunners in the battalion feeding ammo through M60's and M2's as fast as we could. Many times we would be sitting on a stool with the gun on the tripod just swinging it back and forth between our knees. Assuming an average of a half hour of shooting time for every hour spent on the range, at 600 RPM that works out to about 18,000 rounds per hour, at least 100,000 per day. I wouldn't be surprised if I shot up 10 million rounds.
Since then, I've probably averaged at least 1,000 per month (that I had to buy - not Uncle Sam!) for the past 40 years, that works out to about 500,000.