So the US still didn't have an answer to the Soviet AK-47. Eventually an inventor came up with a very light weight .22 caliber rifle firing a modified .222 cartridge (normally used for hunting rabbits and foxes). The gun was small and light so the Air Force adopted it as a gun their aircrews could use if they bailed out. It was fancy, looked high-tech, and used lots of aluminum, the Air Force's favorite metal, so they bought lots of them. Someone showed it to President Kennedy and he thought it was nifty. The US Army despised it. They worked for Kennedy, and he told them to adopt it, so they did, after trying everything in their power to keep using a real combat rifle.
Two different guns there. The Armalite AR-5 was the aircrew survival weapon, a lightweight, collapsible bolt-action in .22 Hornet. The first M16 was the Armalte AR-15, by that time owned by Colt, approved by Curtis LeMay for USAF base security forces.