“It isn’t made to fire in sustained full auto.”
Bingo as they say here at FR. If M4 met the requirements of sustained 400+ round firefights then it would weigh 15-25 pounds like the current inventoried/deployed weapons do that can sustain that volume.
Infantry works as teams. Fire team, squad, platoon, company, battalion, etc...each component has a balanced arsenal of weaponry to meet the missions they are SUPPOSED to be assigned. Sustained fire is the province of a few squad weapons keeping an enemy pinned down while the riflemen maneuver close enough to grenade the bastards. Or for the mortars, arty, or air to respond and end the disagreement in a civilized manner.
For specialty applications like long range (400 meter+?) accurate anti-personnel shots, there also already exists in the US DoD inventory many a fine tool in many fine platforms and calibers.
But what the heck do I know and I don’t feel strongly about it.
I really don't want to settle anything from any distance by myself. I like a whole bunch of stuff going their way topped off by a grenade, mortor, tank round, bomb or shell. Those I believe are the bestest types of fight. In fact I recomend doing house clearing with a tank round. But I'm not in charge.