how much silver in a shekel

The Rambam (Shekalim 3:2) tells us that a shekel is the weight in silver of 300 barley seeds:

ב מניין כסף האמור בתורה באונס ובמפתה ובמוציא שם רע והורג עבד, הוא כסף הוא שקל הנאמר בכל מקום בתורה ומשקלו, שלוש מאות ועשרים שעורה

I always assumed this is some type of average, because given any 300 assorted barley grains you would expect some slight differential in the total weight. Live and learn - Peter Bernstein writes in “The Power of Gold: The History of an Obsession” (p. 24), “Today the carat has been replaced by the grain as the conventional unit of weight [for gold and other precious metals]. Grains of barley or wheat in the middle of the ear have the same remarkable attributes as the carat – a standard weight regardless of the size of the ear.”
I never knew that!