John, this whole thing is making me think a lot harder than I want to tonight. Electric spark? A bit of plasma excited by an electron packet passing through it. A spark created by other energies? A particle raised to incandescent heat that is in the process of oxidizing. Would a grinder strike a spark in a flow of nitrogen, or a welder in a flow of helium?
A third type, maybe Copper sparks? Is that just a bit of metal that absorbed so much energy that it reached incandescent heat, but hasn't created a self sustaining state of 'burning'?
A steel spark will continue to build heat as it burns, as will ignited magnesium, these things generate heat during combustion, as do many bio organic things, or even charcoal, right? But since a spark off of copper doesn't combust, won't that tiny little packet of heat energy dissipate so easily that igniting a piece of cloth should be literally impossible?
The entire question is whether a bullet or other object can dump enough energy onto another surface to blast off superheated particles, don't you think? Oh, heck yes. part of the energy energy of forty grains of powder smashing into steel, no matter what the impacting object is releases a metric fartload of energy and could break off a chip of steel that would absorb enough energy to ignite. But, both lead and copper are too soft and malleable to release that sort of energy, right? Those impacts spread the kinetic energy out both in time and into deformation, right?
One of my most important questions, is why do ferrous metals spark, but most non ferrous metals won't when ground by a super hard stone or ceramic? Here's my thought. Steel has a brittle structure and it is broken off in chips at high speed, and they ignite in air because smashing them off generates so much heat. Iron is also very active with oxygen, especially in the form of steel. Most common non ferrous metals, aluminum, copper, lead, that stuff is scraped off at temps too low to ignite the metal, and ordinarily, if they do strike a spark upon impact, it would cool very rapidly, because it contains very little energy.
Here's something that I just can't fit into my tired head. Brass on a grinder smears and leaves brass alloy dust. Grinding steel leaves oxide. Watch it grind, and you can see those superheated particles literally explode as they pass through the oxygenated air. Particles break up.
So, it seems to me that malleable,not particularly active metals shouldn't under ordinary circumstances strike a spark, observation over centuries kind of supports that. But unusual circumstances that can't be understood by a dolt like me will absolutely allow sparks of some sort to pop off of a malleable metal.
If a person packs enough ke, through velocity, into a lead projectile, say maybe 20,000fps, would you expect it to dissolve into a fireball upon impact with the proverbial immovable object? I would, I guess...