John Gill 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)

They were armed with bows. Much used in war in those times.

and could use both the right hand and the left, in hurling stones, and shooting arrows out of a bow; they were ambidextrous; who could sling stones, or shoot arrows, with either, which was no small advantage to them.

even of Saul's brethren of Benjamin; they were of the tribe of Benjamin, of which Saul was, and so his brethren; and they might be, at least some of them, his relations and kinsmen; who observing the unreasonableness of Saul's enmity to David, and detesting his cruelty, and sensible of the innocence of David, and of the service he had done his country; and perhaps not being ignorant of his divine right and title to the kingdom, went over to him, to comfort, strengthen, and assist him.