There’s a huge difference between a Vietnam era M-16 and a Modern day M4 or M4A1.
Of course, there’s always room for improvement.
Yep - 6-1/2” of barrel length for starters.
The platform is very flexible ... and has a narrow range of “optimal”. Easy to make changes which drive performance to unacceptable, but not hard to adjust back to good performance. Other platforms are much more tolerant of changes, but simply can’t be adapted to what’s needed that the M4 can do. Keep the system balanced and it will work well. The Vietnam era M16 is the recurring prime case: right powder (as designed) would have run the system well, but some idiot decided otherwise without understanding the consequences.
REPORT: The Mystery Rifle That Smoked The M4
http://www.shootingsportsretailer.com/2014/08/20/report-the-mystery-rifle-that-smoked-the-m4/