It's been a while but when I went to the field I'd had 460 rounds of 5.56 M193. Had a lot of the 20 round magazines w/18 rounds each. Some, maybe two thirds was in the cloth bandoleers in stripper clips.
I also carried a 1911a1 but only had two magazines and some loose rounds.
We went to the field for 30-60 days and you never really knew if you were going to be resupplied when you were suppose to. Especially in the monsoon season.
I don't recall seeing the 30 round magazines until I joined the National Guard in '73.
I still like the military 20 rounders better then the 30 rounders.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071