I wielded 14's in basic training. HEAVY LOG compared to the 16. And then packing lots of 7.62 ammo as well was ARG ! Wasn't ever privileged to subject a 14 to what the 16 went through, but talking with vets who did, they had a LOT less jamming issues with the 14. The M1 carbine woulda been better than the 16 in the early years. Short, light, accurate.
The modern AR type base is sooooo much advanced, LIGHTER, and doesn't jam as easily as the early 16's. With all the body armor they have to wear, ANYthing to lighten the load and still pack lots of water and ammo is priority.