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I'm not arguing about any of that . There is a huge amount of kinetic energy in that bullet that is converted into heat upon impact the bullet, smashing up and being deformed absorbs a bit of energy and warms up. The rest of that energy is transmitted to the steel plates as vibration, with that being turned into heat, and even noise. All of that energy is either used up in destroying the bullet, scattering the piece, or is transferred to the plate as heat and some is absorbed by vibration and just dumped into the air. All of the energy can be accounted for, just like tapping that plastic.
Whatever happened to cause a spark hot enough to char paper and set up the chain reaction resulting in a fire, I can't guess, but the bottom line is that your bullet had a lot of potential kinetic energy that was turned into heat by deforming and transferred into the rebar as movement.
We don't have to know how it happened and I'm never going to figure it out. What we know is that your bullet contains some of the energy released by the powder, and that bullet will expend it in several ways that release heat.
If that steel was covered with flakes that absorbed a lot of heat, known as you say, spall, those flakes might have set a fire.
When I was a kid, I thought that brakes couldn't get Hot. I felt a disk, it wasn't hot. So, I didn't get it. But they do, don't they?
I'm not a scientist. I am glad that I can at some point say'huh. I don't get it'. I don't have to understand it to make it true. Gravity works.
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