Conventional Wisdom

  1. ADMIN
    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.

    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.

    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 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.

    Now we’ll look at an instance where the conventional wisdom is correct, but a little ambiguous. Most of us have heard that “sticky extraction and hard bolt lift” are early warning signs of excessive pressure in loaded ammunition and that sensible reloaders should back off a bit. The concepts are easy enough to understand and to recognize, but exactly what’s happening might not be. It’s just “sticky extraction means back off.” Let’s think for a moment about what happens when a cartridge is fired in a bolt action rifle and the mechanism of hard extraction. A brass cartridge case has two main functions. First, it’s a package, holding various components in a durable and waterproof unit. Without this package we wouldn’t have reliable repeating rifles, and the great armies of WWI would not have had machine guns to slaughter each other. Second, and not so obviously, the case is a seal. It’s designed to “fail,” to expand against the chamber wall on firing and to prevent propellant gases from escaping backwards around the outside of the cartridge. It’s a clever design; essentially the front of the case is a sturdy package up to the instant of firing, then it deforms easily to seal the front of the chamber, and all the gas pressure can be restrained by the surrounding steel barrel. The solid case head – much harder and stronger than the front of the case – is NOT intended to fail. When it does, we have a blown primer if we’re lucky, possibly much worse. The case head, unsupported by the barrel, is forced by gas pressure towards the rear and the shooter, and must be supported by the breeching mechanism. That’s the locked bolt and the load on it is around 5,000-10,000 pounds, pushing directly back towards the shooter’s face. That’s not a pressure, that’s a force and it’s equivalent to two or three cars parked on the bolt, which is the only thing keeping mayhem away from the shooter’s face. The force might be reduced a little by the case walls, depending on the case taper and lubrication, but not much.

    Adding flutes to a rifle barrel makes the barrel lighter, more flexible, and has only a minor effect on cooling. Flutes can look nice though.

    All solid materials – wood, concrete, glass, rubber, steel, Swiss cheese – have properties that engineers call a “modulus of elasticity,” and “elongation at the yield point.” In simple terms, the concept is that all materials are springy to a varying degree. It’s pretty clear that a rubber band will stretch a lot before it breaks, and that the stretch can be recovered if the rubber band is relaxed before it breaks. It’s equally true of steel and concrete, at least in engineering terms. They’re just not as stretchy.

    What’s this got to do with hard extraction? When a rifle fires, the barrel swells radially and the receiver stretches. The part of the receiver ring surrounding the barrel gets larger in diameter, and the part surrounding the bolt locking lugs gets longer as gas pressure tries to push the bolt to the rear. At the same time, the front of the bolt is compressing, getting shorter. The chamber is getting both larger in diameter, and longer. Headspace is increasing. During firing the steel parts of the rifle are not stressed beyond the yield point. All the stretch is temporary and is recovered as soon as gas pressure is released – essentially instantly. This isn’t true of the brass cartridge case, at least in the body area. It has “failed,” stressed beyond the yield point and deformed permanently. This is obvious – otherwise it wouldn’t be necessary to resize the body before reloading.

    Now steel barrel and receiver plus brass case all expanded together, but didn’t contract together. The brass contracted less than the steel, the amount determined by the maximum pressure. Normally, the difference is small, and the fired case is a tighter fit in the chamber than it was prior to firing, but it still comes out easily. If chamber pressure was high enough, the contracting steel parts squeezed the case firmly, and instead of enough clearance to allow extraction, we have a tapered case firmly pressed into the chamber. The higher the pressure, the tighter the fit, and the errant reloader may have to resort to brutality to extract the case.

    This badly eroded "fire cracked" section of a rifle barrel shows more chmical attack than mechanical. Photo courtesy of Gradient Lens Corporation.

    We can deal with the conventional wisdom surrounding fluted rifle barrels quickly. First is the all-too-common statement that fluting makes a barrel stiffer. Why anyone would think that removing material adds stiffness escapes me, but this is probably just a result of people not reading the fine print. What is true is that a fluted barrel will be stiffer than a similar barrel – of equal weight. This happens because the fluted barrel must be larger in diameter to be of equal weight, and increasing diameter adds stiffness quickly. The second often seen statement – that fluted barrels cool faster – is true, but only somewhere around the second decimal place. In still air no shooter will ever see a detectable difference. Why? It’s because the flutes run in the wrong direction and are ineffective for cooling. Look at a baseboard convector and notice that the flutes there are vertical, and perpendicular to the length of the hot water pipe they surround. That’s what properly designed convective cooling flutes look like. If you want your stainless barrel to cool faster, paint it black. That will have more cooling effect than flutes, as long as it is out of the sun anyway.

    Wear inside rifle barrels became a concern just about the time rifling appeared inside them. The concern increased when smokeless powders and jacketed bullets became the norm. It increased much more during the twentieth century, first when automatic weapons showed up, later in the century when armies realized that they weren’t made up of riflemen anymore, and tried to make up for a lack of marksmanship with increased rate of fire.

    Rank these three common cartridges - from most to least "overbore."

    I grew up reading that barrel wear was caused by a tremendous blasting effect of hot powder granules escaping the cartridge case and colliding with the barrel throat. The combination of heat and impact literally blasted tiny fragments of steel off the barrel. Initially this sounds plausible. There certainly is fire and violence inside the barrel and there was published data showing temperatures and pressures were highest near the throat area.

    A bit more thought brings up some skepticism though. Are hot powder particles hard enough to erode steel? After all, the Grand Canyon wasn’t carved by running water; it was cut by billions of tiny bits of rock propelled by water. And how badly can steel be eroded by glancing blows? Most of the funneling of burning powder has to happen inside the cartridge case, and hot bits can’t strike the barrel interior with more than a glancing blow at a shallow angle, and even this happens before velocity gets very high.

    I didn’t really question the conventional wisdom here for decades, probably because I didn’t have any alternate explanations. Eventually though, I was employed by DuPont, then the manufacturer of IMR smokeless powders. I had nothing to do with powders, but I did ask some of the people who did about barrel erosion and uniformly the response was laughter at the “grit blasting” explanation. Reality, now supported by published studies (and much simplified) goes like this. Powder begins to burn. Pressure rises and temperatures in the burning gas are very high, thousands of degrees, and the gas is actually a plasma, ions rather than molecules and very reactive. The atmosphere is high in hydrogen, carbon and nitrogen. Under these conditions, minute amounts of the gas plasma react with the adjacent steel, forming new compounds with mechanical properties differing from the steel, generally being harder and less ductile. This happens on a very small scale, only molecules thick, but on subsequent firings the harder compounds cannot expand with the underlying steel, and minute bits flake off, giving rise to a cracked and eroded surface. This effect is greatest in the barrel throat, where both temperatures and pressures are greatest. In short, barrel erosion isn’t a result of mechanical blasting. It’s mostly chemistry.

    The same three cartridges, this time identified. All are drawn to scale, but not to the same scale: they're all scaled to look like 30 caliber. The bottom row of numbers is the relativel "Overbore Index" according to a popular shooting blog. The 222 ranks near the bottom, the 30-06 is in the middle, and the 50 BMG is the most overbore of the 33 rated cartridges.

    Let’s finish with some fun with the concept of “Overbore” cartridges. Parker Ackley coined that term in the middle of the last century to de-scribe cartridges whose capacity he felt exceeded that which could be efficiently used by the powders of the day. He was talking about rounds like the 220 Swift, 25-06 (still a wildcat then), and anything named Weatherby. Today, he would probably add anything called “Ultramag” to his list. People have tried to quantitatively define the concept, for years, but no definition has lasted. “Overbore capacity” applied to cartridge cases is like the Supreme Court Justice who struggled to define obscenity – hard to describe, but “I know it when I see it.” It’s a big case with a little bullet. Here’s a little visual fun – the three cases shown are covered a recent article defining “overbore” in a popular firearms blog. The definition was case volume divided by bore cross-sectional area, and the bigger the result, the more “overbore” the cartridge. This gives “overbore” the units of length, which seems odd (How many inches overbore is your 300 Weatherby?), but the concept sounds reasonable. Cases were rated on a numerical scale from 540 (underbore?) to 1480 (worst of the worst – keep extra barrels handy). All three cases are drawn to scale – if each one was in your hand it would look exactly like it does in the drawing. Take a moment and rank the three in relative “overbore capacity,” least so, to most “overbore.”

    My choices? Well the one on the right looks kind of old-fashioned and to me the least overbore. The left one has a bit more modern, shorter, fatter look (think about that for a while – short and fat is now “modern”) and to me the most overbore, which leaves the one in the middle – in the middle.

    Is there a gimmick to this? Of course there is. While all cases are drawn to scale, they aren’t drawn to the same scale. In fact, they’re all scaled to look like 30 caliber cartridges, with the smallest drawn to an enlarged scale, and the largest reduced in scale. Remember though, that overbore is the relation of body size to neck size, so this change in scale shouldn’t affect your rating. A big case with a little neck is overbore whether it’s 22 caliber or 45.

    Made your decisions? Well, here are the same three cases in the same drawing identified by caliber and with their overbore ratings as given in the article. My choice for the most “overbore” was the mild 222 Remington, rated by the criteria as downright meek, while the one I thought quite reasonable is the 50 BMG, rated the absolute worst of all.

    Another definition of “Overbore” uses something called “Ballistic Efficiency,” which is the percentage of the chemical energy stored in the powder charge which gets converted to kinetic energy in the bullet (as opposed to heat, noise, and recoil). By this measure, cartridges which convert less than 25-30% are called overbore and none of our three test cartridges qualify as much overbore. In fact they’re all about the same, mostly showing up around 29% or so. Still know what “overbore” means? Maybe this is a definition the world just does not need.

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