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Old January 21, 2018, 03:28 PM   #26
briandg
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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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Old January 21, 2018, 03:47 PM   #27
JohnKSa
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Brian,

The article I linked to has a lot of information that may help answer some of your questions and/or provide insight into this issue.

Here's an example:

The test fired bullets into steel targets. One finding was:

"Thermal infra-red video and temperature sensitive paints suggested that the temperature of bullet fragments could exceed 800°C."

A piece of metal at 1500°F definitely has the potential to start a fire in paper or wood, paper or dry grass (all of which have ignition temperatures less than 600°F) , whether or not we agree that the metal fragment meets the definition of a "spark".
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Old January 21, 2018, 06:20 PM   #28
briandg
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Absolutely true, the only question is whether a hot piece of metal has enough heat energy to ignite the particular product. Doesn't take much to start tissue paper or cotton balls smoldering, and just that one spark of 'fire' can create a chain reaction and catch. The biggest difference between just a hot piece of metal and a hot piece of burning metal, such as a piece of ground off steel, or maybe even thermite, is that the burning piece is releasing chemical energy as well as just the stored heat.

there are people who could break this down. not me.

This thing even involves the question of specific heat or heat capacity. Specific heat is the measure of how much energy must be added to a kilogram of a substance to raise that kilogram of substance one degree celsius, iirc. Brass, copper, iron, lead, have low specific heat figures, it takes very little heat to raise that temperature, therefore there isn't a lot of heat release when it cools down. Beryllium, however, has a much higher specific heat figure. It will take four times as much heat input measured in calories to raise that kilogram of beryllium one degree. it may take a torch a minute to heat up a piece of lead but you will need to run that torch longer to burn more gas and push more heat into the metal, and then, you will release more heat when the metal cools.

Stored heat. the way one of my chem professors put it, would you be smarter to put your hand in a cup of boiling water or a quart of water at 120 degrees until the water cools? There is a lot more heat stored in the quart of 120 degree water, so it will burn you worse IN HYPOTHETICAL CIRCUMSTANCES.

sure, the quart will release far more heat. But a cup of boiling water will scald the skin off.

I'm not sure what my point was.
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Old January 23, 2018, 04:15 PM   #29
PolarFBear
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Fire Marshal proclaims--

As a "wrap" to this story. Local Fire Marshal, TBI, arson investigator, etc. have all agreed that a customer shooter did in fact fire a tracer round into the back stop. The back stop was constructed with shredded rubber.
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Old February 1, 2018, 09:56 PM   #30
langenc
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Response to post #3

sawdust in a bullet trap w/ tilted steel backstop.. When in college the ROTC armory had such an arrangement but had SAND in the pit--no flammable saw dust.
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Old February 1, 2018, 10:30 PM   #31
In The Ten Ring
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Is this the right link?

http://wncn.com/2018/01/25/tracer-am...oor-gun-range/

I have never been a fan of the rubber backstop....I would want angled steel such as I saw in a documentary once on guns.
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Old February 3, 2018, 11:24 AM   #32
briandg
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a rubber backstop as spoken would have been a literal backstop, with a steel plate and pit full of shredded tire. The secondary backstop was a set of large sheets of what is essentially tire material, black tire rubber laminated with threads of some sort, possibly fiberglass in many brands, but some use plain thread.

This is just a backstop intended to stop or catch as much backscatter as is possible, there must be a steel plate behind with a pit to catch bullets.

Tracer rounds are hot. Very hot. They are a metal mixed with a binder, an oxidizer, and an element that will add color to the flame. The metal is generally magnesium, I don't remember what the oxidizer is. Magnesium burns very hot and brightly. the binder prevents this oxygen fed fuel from blowing off instantaneously like a flash bulb. the colorant, likely strontium, burns along with the magnesium.

A tracer is essentially no different than a firework rocket or the stars from them. Black powder or another material. the powder is treated with a retardant. magnesium or aluminum are added to burn it brighter. There are many different metallic salts that either in a combination or alone burn off with the high temperature magnesium to add colored light to the hot white light of the magnesium. Sometimes plain iron is added to the flare for color.

So, a tracer intended to reach out for hundreds of rounds was discharged in a single pit, blasting extremely hot gasses into a pit/pile of easily ignited rubber shreds. Then, you have a tire fire. There are other types of steel backstops. a standard angled sheet of steel deflects the bullet downward into a spiral shaped tube. the round tube forces to bullet to run round in a circle and expend its energy as friction. as bullets pile up, there is no need for the thing to spiral around the steel, it's just locked in place and the impact energy is expended against the piled bullets instead of the steel snail as friction.

A truly high tech range like the nra has at one of their facilities has a continuous oil feeder that lubricates the steel. It is obviously a non-flammable oil such as glycols or silicones, something that would be able to withstand the high heat of friction and resist sparking from any steel particles that hit it. The oil's purpose is to reduce friction heat, prevent some lead buildup, and to capture any sparks that are caused by bullets on occasion.
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