An M16 Rifle weighs what, 15 pounds? Full combat infantry gear, 40 pounds? Thats 55 pounds on you. Anyone can carry that. Modern Rifles have incredibly little recoil.
The problem isn’t so much with being able to carry 55 pounds (though this is in its own right heavy [ex. lot of girls complain when their backpack weighs more than 10-20 pounds), but carrying it over long distances. I have done a lot of reenacting with such loads (though more like 30 mostly) and after two hours of fighting, sprinting, and marching, it starts to wear on you - A Lot. This is esp. true when your exhausted, under harsh terrian, in fog of war, and under heat - not many people can take that. Besides, I never want someone in my squad covering me who doesn’t want to fight - this often results in two deaths instead of one.