Albert Barnes Commentary 2 Samuel 21:16

"and Ishbibenob, who was of the sons of the giant, the weight of whose spear was three hundred [shekels] of brass in weight, he being girded with a new [sword], thought to have slain David." — 2 Samuel 21:16 (ASV)

Ishbi-benob - A corrupt reading. The whole passage should perhaps read as follows: And David grew faint. So they halted in Gob (as in 2 Samuel 21:18–19), and there was a man in Gob who was of the sons of the giant, etc.

Sons of the giant - The "giant" mentioned here (2 Samuel 21:18, 20, 22) is "ha-Raphah," from whom came the "Rephaim" (Genesis 14:5; Deuteronomy 2:11). The sons of Ha-raphah, or the Rephaim, are different from the "Nephilim," or Giants (Genesis 6:4; Numbers 13:33). The sons of Anak were not strictly Rephaim, but Nephilim.

Three hundred shekels of brass - About eight pounds. Goliath’s spearhead weighed "six hundred shekels of iron."