The M16 is a fine rifle when maintained. However, the design is almost 50 years old. Other nations around the world are creating new designs and better designs while we redress the same platform over and over and over again.
Sadly, we won’t see any real new effective systems here in the US for a long long time. Thanks to all the short sighted elected officials that killed the innovative firearms industry in this nation.
The culprit is federal law 922(o): prohibition of civilians owning/building any machineguns made after 1986.
Worldwide, the leading edge of firearms development is "lots of tiny holes" a la the FN P90, HK MP7, and several other ground-up redesigns. The application is compact arms which can penetrate soft armor, solved by tiny projectiles in cartridges between the .22LR and 5.56x45, stored in 50-round magazines; the small size penetrates well, and low recoil + full auto + large magazines = lots of holes = sufficient damage via a compact package.
BUT ... we're not allowed to make any. Domestic manufacture of any arms is dwindling, with the only real advancements coming in high-priced long-range single-shot systems like the .408 CheyTac, and is driven/funded by civilian markets. There being no legal market for new full-auto products, and research requiring hard-to-get licenses, we're stuck tinkering with 50-year-old platforms and optimizing ultra-long-range rounds.
Overturn 922(o) - c'mon, Heller practically gave us the template - and you'll see new effective systems appear almost overnight.