ZQ1 (ZQI) 5.56 ammo at Walmart

[imarangemaster]

I think it is also referred to as "Cold rolled" steel. Not armor plate, but fairly hard and used to fabricate plates on machinery, etc., according to my buddy the welder.

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[vintage hunter]

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.