steel jacketed bullets don't do anything worse to hardened steel backstops than copper or brass jacketed bullets.
The jackets are typically so thin that all three materials share similar ductility and will preform the same. The surface hardness of the materials play no role in backstop damage. It's energy and sectional density that determines how much impact damage will be done, and will be done by which ever material has the most mass (the core). Damn near all of these indoor ranges with hardened steel backstops are rated to take multiple 50BMG hits and survive. That's 18,000 Joules of energy with a sectional density of 0.4, and a much thicker copper jacket than typical.
Somehow to these indoor range owners, that is less damaging to their backstops than a 400 joule Tula 9mm steel jacketed bullet with a sectional density of 0.13.
This is the biggest load of crap I've ever heard in the gun owning community. I've heard this before and it's complete non sense.The steel cores and jackets spark heavily on impact and increase the chance of range fires in the traps.
Sparks are not embers. Anybody that's worked in a shop environment can tell you that. Grinding mild steel creates sparks, but no embers that linger on the floor.
The small piece of steel that is removed from the jacket upon impact and turns into a spark never lands on the ground glowing. It loses its glow well before decent. That means it has lost all of its heat and has reached ambient temperature. The smaller a mass is, the faster it sheds heat. The small fragment of steel that glows as a "spark" is only a few microns thick. Given the temperature of ignition of most smokeless powders, it is inconceivable that this so called "spark" could possibly ignite loose smokeless powder particles on the range floor.