Yeah, troops in contact running out of ammunition and nobody thought to put the bullets in magazines before sending them on their way.
They are called 'rounds' - not bullets. D Company carried the standard 3 x 20 round magazines per rifleman plus M60 ammunition. When they encountered and were cut off by a Battalion-sized enemy force they not surprisingly ran short. Soldiers on foot patrol through jungle can only carry so much ammunition along with all the other equipment. That is why they rely on a resupply chain.