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Conventional Wisdom

Conventional Wisdom

By Josh Benin
Mouse Engineering
©Copyright Precision Shooting Magazine

A former colleague once told me that success in Corporate America depended on being able to “move faster than the speed of smell,” meaning that it’s more important to seem right than to be right. This suggests caution around “Conventional Wisdom” – ideas or explanations which are generally accepted as accurate – and which may or may not be.

Conventional Wisdom
An orange dropped into a bowl of water on a countertop demonstrates the "Buzz Saw Effect." The water splashes in the direction of least resistance - radially outward and back along the direction of the orange's travel. The orange was not spinning. Photo from openPhoto.Net

We’ll take a look at a few firearms-related bits of the Conventional Wisdom from across the spectrum, some of which are true, and others which aren’t. We’ll try to be scientific about things, examining if the concepts seem plausible, if data fit expectations and if the whole idea generally holds water.

One of my favorite bits of Conventional Wisdom is the “Buzz Saw Effect,” the idea that a lightly-constructed, high-velocity rifle bullet rotating at extreme RPM, can literally explode on contact, releasing tremendous energy stored in rotation as fragments fly outward. Bullet manufacturers sometimes specify maximum velocities for their products, warning that exceeding those velocities might cause the bullet to fail in flight, and shooters who have let their rifle’s bore fill up with metal fouling sometimes have bullets disintegrate in flight. I’ve never in my life shot a plastic bottle full of water but I have seen it done, with impressive results. Water sprays sideways and bits of bottle fly up in the air. All of this makes the “explosion” idea seem not just plausible, but right on.

Looking at the science, we can calculate some numbers for my 6PPC rifle, using a load of a 60 grain bullet at 3390 feet per second from a 12" twist. That works out to just over 200,000 RPM, not enough to cause the bullet to fail at room temperature, but one would expect a bullet to be weakened from engraving by the rifling and by the temperature rise from bore friction. The rotational speed of the bullets’ surface is just over 200 feet/sec. (not much is it?). The bullet has about 1500 foot-pounds of energy from its forward velocity. And the energy from that 200,000 RPM of rotation is: about 3 foot-pounds, or almost nothing. This is physics’ way of showing that something very small can be spinning like hell and still not amount to much, or mathematics way of saying to be careful when multiplying a huge number by a tiny number. Rotational energy is negligible, and this is not at all comparable to those horror stories we’ve all heard about auto flywheels coming apart. So what about that bottle of water that exploded? Does this mean it didn’t explode after all? Nope, we saw just what we thought.

Conventional Wisdom
Here, greatly exaggerated, is what the receiver area of a bolt action rifle looks like at the instant of firing. The barrel, case, and receiver ring have swelled, typically around .001-.0015" in diameter. The receiver ring has also stretched axially, the bolt has compressed, and the locking lugs and their abutments bent slightly, increasing headspace around .001".

Experimentalists might take a moment here and run a small test. Get a pan of water – the local dog or cat’s water dish will be fine – stand next to it, and drop a small rock into it from about chest height. Water will splash out to the sides and get your leg wet. That’s the buzz saw effect, due to a rock which wasn’t spinning at all. In fact, that water bottle we exploded would have exploded just as spectacularly from a bullet shot from a musket – no spin at all – if we could have hit it.

With an exploded water bottle, a wet pants leg, and some abstract calculations, what actually happened? Imagine a molecule of water, just inside the bottle, right at the bullet’s impact point. The bullet is going very fast (three times the speed of sound), and suddenly the bullet and the water molecule are trying to occupy the same point in time and space, something which nature will not allow. The bullet is beginning to fragment and rapidly transferring a large amount of energy to whatever it hits, water in this case. The water molecule, obeying nature, begins to get out of the way, but is restrained by the inertia of more water behind it and the surrounding bottle. It takes the path of least resistance and water at the impact point begins moving rapidly, splitting the bottle beginning at the front. Water sprays out to the sides and upward. All this happens before the water molecules in the back of the bottle know what hit them – literally. The bottle bursts under the force of the water leaving, and is forced in all directions. Downward force is resisted by whatever originally supported the bottle, left and right forces tend to balance each other (after all we hit the bottle dead center), so the remains of the bottle head generally upward and backward, which is just what we see in real life, and has nothing to do with bullet rotation.

Careful study (maybe more that is merited) of high speed video clips at http://www.youtube.com/ watch?v=QfDoQwIAaXg shows that when bullets hit, the first thing happening is bits of the target leaving in a hurry – at a speed much higher than the rotational surface speed of the bullet, and in the direction of least resistance, usually back towards the shooter. Physics and observation agree, and the conventional wisdom isn’t too wise here, although I’ve been hearing about the buzz saw effect my whole life.

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