Albert Barnes Commentary 1 Chronicles 12:33

"Of Zebulun, such as were able to go out in the host, that could set the battle in array, with all manner of instruments of war, fifty thousand, and that could order [the battle array, and were] not of double heart." — 1 Chronicles 12:33 (ASV)

Expert in war... - Rather, this refers to men “equipped for battle with all kinds of armor, who could form a battle line with an undivided heart,” meaning they excelled in their weapons and equipment. The writer notes in each tribe the quality in which it was most admirable.