so the question...how thick?
My gong is made out of 3/4" mild steel. I am shooting a .338wm. At 300 yards it is making dents about 1/2 way through with a 250grn bullet.
The boys are going to make me one out of a hardened 3/4" steel so it doesn't dent so badly.
I would not use mild steel again.
FWIW I would exercise extreme caution shooting a steel plate at less than 200 yards.
