Armor is deceptively simple in its construction. There are other threads in this forum where we discuss this but I repaired armor for THREE different armored car companies (whom I worked for transporting money too) and I can tell you that a NIJ Level 3(Rifle) van contained 3/8" case-hardened (on its face) mild steel, 1" of cork or styrofoam and the 16 gauge outer skin of a Ford E350 Econoline van. This combination WOULD STOP rifle rounds.
Here is a video made by Demo Ranch of a 1/4" MILD steel plate "flex sealed" to a thick (1") rubber mat used in horse stables with concrete floors stopping EVERY handgun round out there COLD.
https://youtu.be/uj1cm8WUGfg
swag