Even in combat it is rare to see a solder have more than 200 rounds at any given time.
Where did you read that at?
Tis true the "standard" BII of the 11B is 210 ( 6 mags in carriers +1 in the well)- that's for immediate access on their person but the supply train (or ruck or can or whatever) is close by.
Those who have been in an intense sustained firefight will tell you just how fast that can get burnt.