Hi Butler. The following is from the NRA Silhouette Handbook.
Note that their recommendations assume the use of High-Power rifles, which use the same silhouettes as BPCRs. I'd assume, and you know what they say about assumptions, that a softer steel would be OK if limited to BPCR. Since I do not have any direct experience on the type of material and thickness some use for BPCR maybe others can comment. In most cases I understand they follow the NRA recommendations which state:
"It has been found that some low strength steels and some high strength steels are not satisfactory, because of the extreme damage done to then by high velocity bullets" The NRA further states the best steel to use is described om the ASTM specification A-514. It includes several which varies from about a BHN (Binell Hardness Number) of 320 to 400. The silhouette handbook lists several recommended steels, one of which is AR320.
The silhouette thickness is 1/2" for the chicken and pigs, 3/8" for the turkeys and sheep. The feet (stand) are to be made of the same material as the silhouette and are 3"x4" for the chickens, 2"x4" for each leg of the pigs, 3"x3" for the turkey, and 4"x5" for the sheep.
Wayne