Albert Barnes Commentary 1 Chronicles 12:2

"They were armed with bows, and could use both the right hand and the left in slinging stones and in shooting arrows from the bow: they were of Saul`s brethren of Benjamin." — 1 Chronicles 12:2 (ASV)

The skill of the Benjamites as archers is noted in 1 Chronicles 8:40 and 2 Chronicles 14:8. Their proficiency in using the left hand appears in the narrative of Judges (Judges 3:15, and marginal reference), where their special excellence as slingers is also noted.

Even of Saul’s brethren (compare 1 Chronicles 12:29). Even from Saul’s own tribe, some separated themselves from his cause and threw in their lot with David.