[applesmasher]

I have been inspired by talk of homemade bulletproofing to consider the question of how it might best be done.

The basic challenge behind bulletproofing is that you have a fair amount of energy coming in on one narrow front, and you want to absorb, disperse or redirect that energy without greatly affecting what you want to protect.

Plastic deformation is one option, with a bullet deforming as well as a plate of steel (for example) deforming. Both items convert the kinetic energy into the work of deformation, and when the energy is exhausted penetration stops.

Dispersion is another option (although we can presume that in most cases significant plastic deformation of a bullet will occur as well) when a bullet hits something like water, gel, or even sand. The part which receives the impact transmits energy in all directions, rapidly reducing the concentration of incoming energy.

Deflection, often interpreted as a ricochet, changes direction but doesn't necessarily greatly reduce the projectile's velocity, so in the absence of something else to take the impact it is probably not wise.

I had some notion of slips of steel sandwiched in a quilted kevlar (or other aramidic cloth) garment, but then I thought of layering steel scales to get good coverage, and then my mind filled with a million ideas.

Any thoughts?

 

[briandg]

You cannot practically bullet proof against rifle rounds. I've seen .223 blow holes through 1/4 inch steel. It's not going to be terribly dangerous after expending all that energy and being deformed, maybe even destroyed, but it points out that you can't do it on a large scale. You can't use kevlar wallpaper.

The ceramic plates and vests used professionally work about as well as they can against rifle rounds.

On a structure, all you can do is build the thing solidly and the energy will be absorbed. Cast concrete will do it, but almost anything can blast through a cinder block and cause injury or death. A foot or two of compressed dirt. Two inches or so of shredded tires. The Earth ship that the hippies make? Dirt pounded into old tires that are stacked to make a hovel? Sure, 2 feet of dirt will stop a bullet, and the last layer of tire will help even more.

 

[briandg]

Just FYI, one of the original bullet proof vest designs included mail and fabric as you suggest, some were canvas filled with sand or lead shot, some were plates, there were a lot of approaches, but they all depended on the same thing. Trapping the bullet by mechanical means, the heavy canvas, and a solid or near solid substance that would absorb energy by being moved by the impact.

Like a BB gun and a can of water. it may penetrate the first layer, say the canvas, but then it has to push aside the water, and then the final layer is another chunk of steel that will maybe bounce it back through elastic collision. In this process, the all through the penetration, there will be energy absorbed just because the BB has struck that first layer of steel and shaken the entire can full of water, and will be stirring it up all the way through.

IIRC, the nra range has a snail type backstop, one inch steel, with a continuous oil flow down the side. It can handle everything all the way to the big express rifles. It uses the energy on the secondary function. The impact on the angle transfers minimal energy, the rest of that energy is drained as it spins inside the snail chamber, both through friction and by having the constant pressure against the steel plate that the spin chamber is made of.

When I was young, I built my own BB trap out of an inner tube. It was simple. the bullet couldn't penetrate; it struck the rubber and the entire load of energy was absorbed by popping that sheet of rubber.

 

[applesmasher]

The way I was thinking, penetration is a function of a number of factors. Sectional density and momentum are big ones.

So in principle, if I could somehow catch the entire momentum (this is not realistic) and distribute it across someone's entire frontal profile, even a .50BMG would mostly just feel like a shove. Uncomfortable, but in no way lethal. Alternatively, if I could hold a five pound cast iron cannonball directly in front of the round, it would probably not penetrate at all, and the motion of the cannonball would be easily handled.

So let's say I had a couple of material technologies:

• An easily distorted but substantially impenetrable tissue or fabric. By itself, not very helpful because a bullet on hitting it would simply drag the material into my tender, tender flesh before it comes to a halt. Still a nasty wound from my perspective.
• An ablative item which shatters, or flies around like birdshot on being hit by a bullet, but which in the process sucks up a lot of kinetic energy by turning it into work. Not great by itself, but maybe it will convert a deep wound into a shallow one.
• An elastic layer which can catch and absorbe light impacts, or by plastic deformation greatly reduce penetration.

Somehow combining these technologies sounds more useful than bulking up any one of them. First, a layer of the cloth over the ablative layer, holding it together (kevlar and birdshot, perhaps) over another layer of the cloth, which then encapsulates a plate of the third (suitably tempered steel, perhaps). The bullet flies along, hits the kevlar, tears it up, but also flattens a lot and loses a bunch of energy hitting the birdshot. If a spitzer bullet, it might even start the bullet yawing. The birdshot and mangled bullet tear up the second layer of kevlar, and deliver their momentum to a steel plate which buckles and distributes the bullet and birdshot's reduced momentum over a wider area, perhaps four square inches.

If the bullet was a .50BMG three feet from the muzzle, it wouldn't help much. If it's a .17 varmint round, it might not even raise a bruise. A .44 Magnum might crack a rib but I don't think it'd penetrate.

Anyway, that's my line of thought.