For example, the issue of the M16's reliability problems, which seem to be inherent to its design, have never been addressed. Nor, for that matter, has its fragility. Furthermore, while the effectiveness of the 5.56mm NATO round is quite good, it too has been around for quite some time. Personally, I'd like to see someone come up with something between it and the 7.62x51mm NATO.
Or, we could just admit once and for all that a single weapon is unlikely to do all things well, and develop specialized ones. It worked during WWII, when the typical squad would have a mix of M-1 Garands, BAR's, and Thompsons and carbines. Seems to me it worked pretty well, considering the results.
Besides, not all the services have the same requirements. My own, the Navy, still keeps M-14 rifles in its inventory, for example. H&K MP-5s as well.
Just one Sailor's opinion.
They did. Its called .243 Winchester. A high performance cartridge--100grain bullet with muzzle velocity of 2900fps. It is a necked down .308/7.62 case with a 6mm bullet. Short maximum length of 2.7-in.--ideal for automatic weapons. Light recoil. Women and children have been knocking off white tail deer with this for years.
Sounds like half of a case for 7.62x39 ... urk!