Shooting at different types of steel has a differing effect.  Steel hardness is rated on the Brinell Scale and is indicated in Brinell Hardness Number (BHN).
Mild Steel - 120 BHN
Not suitable for any calibre beyond rimfire.
AR400 - 400 BHN
This is an excellent choice for small bore rifles at 300+ yards such as .243", .308",  etc.
AR500 - 500 BHN
Well suited for magnum calibres such as the .338" Lapua
Mild Steel - 120 BHN
6.5mm Creedmoor vs. 3/8" mild steel at 200 yds.
AR400 Steel - 400 BHN
.338 Lapua Magnum vs 3/8" AR400 at 200 yds.
AR500 steel - 500 BHN
.338 Lapua Magnum vs 3/8" AR500 at 200 yds. Just the paint sir.... just the paint.

WARNING

Shooting can be a dangerous activity. Prairie Precision Steel shall be indemnified of any claim of personal injury or damages occuring during the use of its products.