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Rocky’s Reloading Room |
What the Heck is “Overbore?”
Ask a shooter what “overbore” means and he may have flashbacks of a PTA picnic. Ask him to explain it in shooting terms, and he may go just as blank.
Much has been made of the term “overbore.” But what does it mean? Some point to huge cartridge cases holding little bullets as examples of overbore cartridges. They may be correct, but how do we know how big a case with how small a bullet represents a normal combination and which is overbore?
In other words, how can we quantify overbore?
Since the term “overbore” does not appear in any ballistics textbook or engineering manual (at least as far as I am aware), it is open to discussion. Shooter, case designer and fellow gun scribe Ken Howell is several times smarter than I am about this stuff, so I’ll base my definition on his work. In his mammoth tome “Designing and Forming Custom Cartridges,” Ken spends a good amount of time discussing case efficiency, or bore capacity. He in turn quotes other authorities and comes up with a figure of about 25-30% of powder efficiency as the limit of bore capacity.
By that, he means that any cartridge that delivers less than 25% of the theoretical performance of its powder charge is over its bore capacity. If single-base, extruded powder has a theoretical energy content of about 175 foot-pounds per grain of powder, then an efficient cartridge will produce about 44 foot-pounds of bullet energy per grain of powder. (The reason why the powder only delivers about 25% of its energy to the bullet is that all the rest is taken up by lost heat, friction, the work of engraving the bullet into the rifling, expanding the brass case, and other things.)
In theory, an imaginary cartridge that uses 50 grains of single-base powder has a potential of 8,750 foot-pounds of bullet energy. Operating at good efficiency, those 50 grains can actually deliver only about 25% of that due to all that lost energy. Our imaginary round should churn out about 2,190 foot-pounds. That number is irrespective of bore diameter, bullet weight and velocity, although all three are obviously involved. So is bore length insofar as it affects velocity, but again, that’s out of our immediate control. Whether it launches a light bullet very fast or a heavy bullet somewhat slower, the total muzzle energy should be just under 2,200 foot-pounds if the cartridge is an efficient one.
So, says Ken, a cartridge that uses 53.5 grains of powder to launch a 210-grain bullet at 2700 fps and delivers about 3400 foot-pounds of muzzle energy delivers about 35% efficiency – and is an excellent round indeed! He actually achieves this efficiency in a 338-06 wildcat. That’s a good one!
On the other hand, a cartridge that uses 72.0 grains of powder to launch an 87-grain bullet to 3600 fps delivers about 2500 foot-pounds of energy. This works out to an efficiency of only 19.8% - pretty poor. This also is a real cartridge, the .257 Weatherby, which is widely recognized as a prime example of an overbore cartridge.
So there’s one definition based on efficiency. There’s another way to describe it.
In addition, the concept of an overbore cartridge can be related to the ratio of its bore diameter to its powder capacity. These days, we generally measure powder (or case) capacity by means of its total water capacity. We do this because gunpowder is an inexact item. It varies in density, kernel size and shape. It would be difficult to get an exact measurement because no two charges of powder ever settle exactly the same, and there is always some amount of air between the kernels. Water, on the other hand is extremely uniform and always packs the same (barring bubbles). So we use water capacity as a more uniform method of expressing powder capacity when we measure cartridge cases.
This leads me to a caution. When we say in the text below that a case holds so many grains of water, it does NOT mean that such a weight is an appropriate powder charge! None of the following is loading data in any way, shape or form!
So, a given ratio of water capacity to bore diameter can be thought of as the threshold of efficiency. In other words, of overbore. But what’s the ratio? Ken didn’t get quite that far in his discussion, so here is where I strike out on my own.
By comparing energy figures of a number of cartridges versus their potential energy, based on powder charges, it soon becomes apparent that the larger the bore, the more potential efficiency is present. That is to say, each small increment in bore diameter produces a much larger jump in efficiency per powder grain. It’s a fairly linear progression, but it is based on the square of the change. A bore two times as large has about four times the potential efficiency.
So then it becomes a simple matter to express our “overbore ratio” in a formula. Simply stated, a cartridge is overbore if its case capacity in grains of water is greater than the bore diameter squared times 1000. Or as a formula:
Capacity > (bore dia x bore dia) 1000.
The constant 1,000 is used simply to get rid of the decimals that result in the bore diameter being squared. Here are some examples:
In a .17-cal cartridge, the largest case not overbore will have a capacity of about 29 grains of water. (.172 x .172=.0296. Then slide the decimal place to 29.6) Cases that hold more water will certainly be faster, but they are not gaining that speed efficiently. The 17 Remington holds about 26 grains of water when full. (We will disregard the space lost by bullet seating, since that is a variable out of our control.) The wildcat 17-223 wildcat holds almost exactly 29 grains of water. All other common rounds in .17-caliber have smaller capacities, which helps to explain why .17-caliber wildcats on larger cases are uncommon: they are wildly overbore.
In the .22-caliber, a case capacity of about 50 grains is the threshold between efficient rounds and overbore ones. The wildly popular 22-250 case holds 45 grains of water and the hotrod 220 Swift holds 48. Among the common .22-caliber wildcats, those based on the 308 Winchester case are about the limit of bore capacity with their 52-55 grain water capacities.
Bores of .243” have an overbore threshold near 59 grains, while those of .257” can use cases with 66 grains of water capacity. This is in the 6mm Remington/257 Roberts territory, making them almost perfectly designed cartridges. Makes you think some of those old codgers weren’t so dumb after all, doesn’t it? And they did it without calculators or computers, too!
How does the 257 Weatherby stack up with this definition? That case holds 84 grains of water, which is about 27% greater than our “ideal” 66 grains. Still wildly overbore, just as in Ken’s definition.
And so it goes. The ever-popular 7mm, with its .284” bore, can use cases up to about 80 grains, while the .30-caliber peaks at about 95 grains of water. Notice that the .300 Rem Ultra Mag have a case capacity of just over 106 grains of water, which clearly makes it an overbore round, as compared to the much more efficient .300 Winchester Magnum, which holds about 88 grains or the classic .300 Weatherby which holds 98. The current extreme is the cavernous 30-378 with a water capacity of more than 127 grains of water! Overbore? I’ll say!
Now how about that .338-06 wildcat that Ken likes so much? Remember that the round achieved about a 35% efficiency using Ken’s definition. In .338” bore diameter, the maximum case capacity calculates to 114 grains of water. The .338-06 holds 67 grains of water. Again, it achieves its performance with remarkable efficiency. It’s also considerably underbore, so the two definitions coincide again.
I’ll jump over a raft of other bore diameters and finish this list with the maximum bore diameter allowed by law, the booming .50-caliber. Here, a case has to hold more than 260 grains of water to be considered overbore. The fabled 50 BMG case holds 290. A tad overbore perhaps, but as usual, John Browning hit it pretty much right on the noggin when he sketched this one out.
So if you’re thinking of designing your own personal forked-lightning wildcat round, just stop and see where it falls in respect to its bore diameter. If your proposed case capacity exceeds our arbitrary threshold, you’ll get high velocity, but it’ll take extra hands-full of powder to get it. There goes your efficiency. And along with fewer reloads to the pound of powder, you’ll give up barrel life, case life, and manageable blast and recoil as well! If you don’t mind those costs, go for it. But if you’d rather get the most performance for the least amount of powder (and recoil, and blast and barrel wear!) then stay below that hazy threshold we call “overbore.”
I’ll conclude with an observation made by Mark Twain, which I’ll paraphrase. Twain said that if a man wants to carry two cats home by their tails, by all means let him. He’ll learn things that he might not have otherwise even guessed, and the experience will be one he’ll not soon forget!
And so it is with cartridges. If someone wants the one that’ll give the highest muzzle velocity with a given bullet, no matter what, by all means let him.
Rocky Raab Copyright 2001 |