Thoughts?
AR Family (M16A2 ands M4) has had serious design flaws forever, but getting procurement changes made through the Infantry Center is like trying to get an intelligent bill that positively effects citizens through Congress. Very byzantine bureaucracy --- the kind Chinese proverbs warn against.
The H&K 416 piston recoil design, with one simple design change over a direct gas piston, is much superior.
Also, the rifles as issued are very rear garrison in their design. The416 comes with rail mounting system as a standard; military AR family does not. Many soldiers spend their own money on AR modification parts to equip their rifles and carbines they use in theater: combat slings, better magazines than issued, rail mounts, bipods, combat sights and optics, mounted lights.
5.56mm round is light. It may have good accuracy, minimal recoil, and light weight to allow a good basic load of ammo be carried, but it does not have the penetration often needed, and it's tumbling round means that light vegetation can throw it off it's intended trajectory. Army is very single minded on the continued use of the 5.56 round, even with plans to move to telescoped and caseless ammo loads. Using the 5.56mm round in squad automatic weapons like the M249 makes a bad situation worse when a small unit needs a heavier punch somewhere in it's ranks.
Many operators, by some accounts, have been adopting the 6.8mm round as a good compromise between the virtues of the 5.56 and 7.62mm rounds.
9mm is useless in a combat environment. .40mm, 10mm, .45... any would be better than the 9mm.
Special Operations Groups tend to choose their equipment in house, and they take what works well for them in the environments they work in. Weapons, clothing, equipment... their whole kit. The common soldier is stuck with what has been decided in the Pentagon, PEO Soldier, and the Infantry Center, as well as Congressional input and lobbying from big defense contractors have decided that the solider needs.
What to know what works well? Look to the operators and PMCs. Not what the common soldier gets.