M4s, and to a certain extent even the full size rifles, tend to have issues with dust. This has led to a focus on piston guns, different gas systems, etc. As a "just in case", the Marines even developed and issue a nice new bayonet.
Old A2s in the hands of rear area troops didn't do that well in the early days of the current wars. For reference, see the Jessica Lynch incident.
and all attempts to use the 5.56 round as a machine gun round have failed. .
The M249 killed a fair number of Somalis, and killed more Iraqis and Afghanis. Partial replacements for it in the SAW role being considered don't change the caliber, they just attempt to become even more light and handy.
We don't use the marksmanship in this day and age and most firefights consist of spraying the area the enemy is in - therefore, more ammunition is required. .
It is actually just the opposite for rifle and carbine armed soldiers and Marines. Especially the more experienced ones, they tend to be very careful about when they fire and marksmanship is quite good- not only in the Marines with their A4s, but also in the Army. Issue of optics is now pretty much universal, and for a time the Marines had so many precise head shots that their was a brief investigation to determine whether they were simply executing prisoners. They weren't. The optics made it just that effective to use their rifles.
The last A2s are being retired from Parris Island this month, fully replaced at last by A4s, but the recruits still learn their A4s with iron sights.
The Aimpoints, ACOGs and EoTechs, particularly when magnified make hits on man sized targets relatively easier. Semi auto fire is normal, most rifles still only have three round burst selectors anyway and these aren't the most useful thing.
There are also DMR variants of the M16 and even M4 in service.
I ran across the shot to hit ratio for the M-16 in our troops' hands some time ago, it wasn't a performance to brag about.
That was the Vietnam era percentage of rounds fired. Things have changed a bit. An M16/M4 is problably more likely to produce a hit than any previous individual weapon.