depends on the gun, distance and bullet. A little .380 or .22 can be stopped by heavy plywood at 25 feet or so. A .357 or .45 will punch a hole through it or sheetmetal at twice the range. If you ever watched a tv show like the "A Team" a little corrugated roofing did the job, but most high power air rifles at 25' can puncture aluminum or at least dent if not puncture sheetmetal at 25'. 1/4 inch steel plate would work unless someone had armor piercing shells, but the deflection of bullet would still be as deadly if not caught by something like wood around the edge. If you have the foolish thought of making body armor, it would still hurt. The best way to stop a bullet is dense composite layers of material which asorbs the energy of the bullet, which is what flack jackets do. So if building a backstop, I would put at least one layer of 3/4 or 1 inch plywood over the steel plate, two would be better. The wood would also allow you something to pin targets on. Oh and the looser they are (such as larger across & height) the more energy will be asorbed by the layers, where a tigh,t heavily supported, rigid 2'x2' target will allow the bullet to penetrate further.