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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