Well, I said I would ask the World War II Living History Association for you, and this is what my brother (their pyrotechnics/safely officer) has come back with:
"As always with these things, there is no one answer. A patrol which stays out for several days would need packs, food, warm clothing etc. Fighting patrols will need lots of ammo, grenades, 60mm mortar, bazooka if there are tanks around.
Reconnaissance patrols are only gathering information so will
take less of that but might bring the SCR-300 radio if one is available, digging tools, binoculars.
The rifle belt holds 10 x 8-rnd clips for a total of 80 rounds, but most people would have a bandolier of 6 x 8-rnd clips as well. BAR belts hold 12 x 20-rnd magazines.
Add on a canteen, bayonet, entrenching tool, first aid packet, wire cutters (maybe), compass, mapcase, fighting knife, flashlight, grenades to be distributed around the squad, mortar bombs (shells if you’re American), one of the various types of field packs, ammo boxes for the M191A4 machine gun if they take one (though it’s probably too heavy), K-Rations or at least a D-Bar, sweater, gloves, scarf.
Not everyone has got all of these but most of it would be passed around amonst the squad.
The very smallest soldier always seemed to end up carrying the heaviest weapon!"
Good luck from a long retired FRPGer…