AR stands for abrasion resistant and the number is the brinell hardness .

It is a grade of steel that is typically used in mining / earth moving applications . Its hardness makes it work real well for centerfire rifle targets . It can be cut and welded like normal steel but applying heat to it screws up the temper so it isn't recommended for target applications as it leads to premature wear and holes . Quality steel targets are cut with a water jet or laser .