A few thoughts:
Most US soldiers in the Vietnam war could expect resupply quite easily, and so had a tendency to reduce the amount of ammo actually carried at any one time.
However, LRRP's and other SOG units, operating "illegally" either in the north or in neighboring countries, could not expect easy resupply, nor could expect timely air/arty support...and were basically totally "on their own". They would carry anywhere from 22 to 30 magazines of 20 rounds each (the early mags for M-16's and XM-177's). The extra mags would be used almost like instant claymores, as "area denial". Basically, upon contact everyone would do mag dumps and peel back away from the contact, then set up counter ambush, again doing mag dumps, until they could extend and disengage the contact. Basically most of the ammo was dumped into bushes, blind, to make noise confusion and deter the Commies from thinking of following them. This was not your standard carefully aimed marksmanship one round at a time, more “burrrrrrrp, mag change, burrrp”
Brits in the Falklands mostly had the FN SLR and specops usually used M-16's or CAR-15's. The guys with SLR's found from direct attacks that they were expending so much ammo so quickly, that the "battlefield pickup" became common in one particular night raid (sounded like one hell of a fight to me, Rolland AT missiles used on Argie machinegun posn's). Many of them would be seen the next day with two FN's: the one they were issued, and the Argentinian one they captured the night before!
But their limit really was the weight of the 7.62Nato ammo itself...you can't really carry much of it before you get seriously overloaded. In those days a typical combat load was 100 rounds, meaning one mag on the rifle, four mags in pouches.
Then Grenada happened: with little time to prep, the airborne troops were to jump onto the sole airfield, and expected the mother of all firefights just to get off the open field. So they jumped with a massive ammo load...and upon securing the airfield, rounded up the “extra” ammo so they could actually walk about the island!
Just thought the history might be worth considering.