It was 5 magazines of 20 rounds, now probably 5 magazines of 30 rounds - 150 rounds available. Even with a couple extra mags, it wouldn’t last long. Mechanized infantry has the means of acquiring and carrying a lot more - the tracks can hold a lot.
LOL I wouldn't really know. I went through IOBC (Infantry Officer Basic Course) as a mech-man but my duty station was the 9th Infantry Division, one of the last remaining "light" divisions in the army. For awhile I was a TOW missile platoon leader, but the TOWs are said to mean "Tired Of Walking!" Eventually we became the army's High Tech Test Bed and got to test the SEAL Dune Buggies and the now common Humvee.