I have a steel one also. I could hit with a mini sledge. I don't know how you would rate it though.
do this with a helmet you don't mind testing.
step 1: find a local gun club
step 2: find shooting range
step 3: start from the smalls caliber round and work your way up( before you move up to the next caliber you should check the helmet to see if it went through or it stopped the bullet.)
step 4: post results.