You've never carried an M-60, have you?
It weighs a lot more than 19 pounds (more like 23 pounds when equipped with a sling, bipod, and a 100 round belt of ammo). Add another 13 pounds for each replacement barrel and a lot more for additional ammo. During training, I had the "opportunity" to carry an M-60 for several days in the field. I would've gladly traded it for something more compact and lighter.
How about the M249 Squad Automatic Weapon. With bipod and a 200 round belt of ammo, the SAW weighs 22 pounds.
Even an M-16 with an M-203 grenade launcher and night vision system weighs more than most people think.
Even the basic weight of the classic Browning Automatic Rifle weighs over 19 pounds. Tens of thousands of them were carried in combat quite successfully.