Adam Clarke on 1 Samuel 17:5

He was armed with a coat of mail - The words in the original, שרון קשקשים shiryon kaskassim, mean a coat of mail formed of plates of brass overlapping each other, like the scales of a fish, or tiles of a house. This is the true notion of the original terms.

With thin plates of brass or iron, overlapping each other, were the ancient coats of mail formed in different countries; many formed in this way may be now seen in the tower of London.

The weight - five thousand shekels - Following Bishop Cumberland's tables, and rating the shekel at two hundred and nineteen grains, and the Roman ounce at four hundred and thirty-eight grains, we find that Goliath's coat of mail, weighing five thousand shekels, was exactly one hundred and fifty-six pounds four ounces avoirdupois. A vast weight for a coat of mail, but not all out of proportion to the man.