In 1980 I was working for the Department of the Army as an Ordnance tech with small arms ammunition among my taskings, then over to the Navy Fleet Logistics Support Division into small arms specifically, all this while the military Joint Services Small arms Acquisition program was ongoing, In some instances we attempted to improve existing equipment, evaluated commercially [COTS] available designs as replacements, and sometimes tinkered with new design concepts ourselves. The M16A2 was one result of our efforts and the M9 Beretta pistol another; so too were the Mossberg 590 shotgun as used by the Navy, and the Ruger security-six revolver that replaced 1940s Colt and Smith & Wesson sixguns used by base security forces. There were other goodies, but those were the big ones.
Both loadings were meant for the 20-inch long barrel of the M16A1 and M16A2 respectively. With the widespread use and adoption of M4 carbines with 14.5-inch barrels, the ammunition performance suffered; one answer has been the 77-grain Special Purpose MK 262 Mod 1 for the last 5 years and another has been the M855A1 semiarmor piercing ammunition mostly found in SAW lmg belts, though it'll work in any M16 family weapon that has M4 guide cuts on the barrel extension.
In the family rifle racks, you'll find six M16 family rifles: An M16A1 configuration that would be immediately familiar to any Vietnam grunt from around 1970 on, it gets M193 ball; an M16A2-type rifle serves as my match rifle and it has had *some* match work done to it. I have a British L119A1 configuration AR carbine with 16-inch barrel and dotscope. My son prefers Kalishnikov's but grandson likes both, and his AR is an M4 clone, 1:9 and 16-inches, with 62-grain SS109 ammo and a 3-9x optic. Milady also prefers her Kalash, though hers are Romanian AK74 receivers with 5,45mm barrels. She does run one AR, though, an AR pistol that fits in her map case, with Hornady TAP ammo. Number six is our spare and loaner, another M4 clone with Elcan 4x usable by any guest who's served in the US military since 1967.
I like MagPul Gen III magazines and Brownells aluminum GI mags. Singlepoint slings on 3 of the 6, GI straps on the A1 and A2, and none on her AR pistol. Usual lube is Break=free CPP, graphite in winter.
Thank you for the detailed response.