Probably just plain old A-36 carbon steel. It's the lowest grade and most readily available structural steel in the U.S., like Jim said not very hard. I would expect M193 to go through 1/4'' thick piece at 80 yards from a 20'' tube.
Nato test requirements for a 5.56mm penetrator was that it had to penetrate a mild steel plate 3.5mm(.138'') thick with a carbon content of .1% to .2% and a Rockwell hardness of B55-70 at a distance of 573 meters.
