Well, maybe. Many of the problems with backstops are the result of cratering, which is what happens when a high velocity bullet strikes the steel. The energy turns to heat and a shallow depression is actually melted in the steel plate. This causes bullets striking the plate to bounce unpredictably, even to come back at the shooter.
Er, that was disproven back at the turn of the century. The turn of the 19th century to the 20th.
Let's look at the facts. Iron melts at 1535 degrees C, steel a bit higher, depending on alloy. The specific heat of iron is .444 J/g/K. Iron's heat of fusion is 13,800 J/mol, or 247.1 J/g. In other words, to melt 1 gram of iron at 25 degrees C (room temperature), you must first apply 670.44 J of heat energy to get it to the right temperature, then an additional 247.1 J to actually melt it. That's for a single gram, about 15.4 grains. And kinetic energy does not get turned into heat energy with 100% efficiency with impacts, far from it. The vast majority of the energy involved is used to deform the bullet and the steel (directly, through impact, with no melting involved).
In other words, you may as well be saying "whacking a tree with an axe puts a gash in the tree because the kinetic energy of the axe is converted to heat and burns away some of the wood."
In reality, bullets and axes both put dents/holes in things as the result of a single mechanism. Pressure. PSI. Kgf/cm^2. Newtons. Not energy, not momentum, not melting. Pressure, the result of acceleration (in the case of impacts, a negative acceleration) being applied to a mass.
The reason why steel only deforms due to high-velocity impacts is very easily explained. Pressure, and tensile strength. Tensile strength is the amount of pressure which can be put on an object without deforming/breaking it. For AISI 1080 steel, heat treated to a hardness of 40 on the Rockwell "C" scale, the yield tensile strength is 126000 psi, and the ultimate tensile strength is 184000 psi.
Yield tensile strength is the maximum pressure which can be applied without permanently deforming the steel; any bullet which generates under 126000 psi will not even dent heat-treated 1080 (though it might fatigue the metal, weakening it without bending it). Ultimate tensile strength is the force required to tear the object. So a bullet that creates more than 184000 PSI will likely punch a hole right through a 1080 backstop, or at least crack it.
Unfortunately, pressure is pretty hard to calculate, since it's more or less impossible to measure the deceleration of a bullet as it strikes something, without extremely expensive equipment.
