Jamieson, Fausset and Brown on Judges 3:31

31. after him was Shamgar--No notice is given of the tribe or family of this judge; and from the Philistines being the enemy that roused him into public service, the suffering seems to have been local--confined to some of the western tribes.

slew ... six hundred men with an oxgoad--This instrument is eight feet long and about six inches in circumference. It is armed at the lesser end with a sharp prong for driving the cattle, and on the other with a small iron paddle for removing the clay which encumbers the plough in working. Such an instrument, wielded by a strong arm, would do no mean execution. We may suppose, however, for the notice is very fragmentary, that Shamgar was only the leader of a band of peasants, who by means of such implements of labor as they could lay hold of at the moment, achieved the heroic exploit recorded.