Home Forums Hunting & Shooting Ask The Gunwriters The meaning of "Overbore"

Ken,
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<br>I know that the term "overbore" gets applied to a number of rounds.
<br>However, if faster powders are used, doesn't that change a cartridge from "overbore" to "just right" or even "underbore" (if there is such a term)?
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<br>This concerns a .257STW project I am involved with. I want to use 115gr. bullets at speeds higher than available with the .25-06 or .257Wby., say around 3400fps.
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<br>It's important that pressures be kept down, if possible, as this round will be used in Africa and the American west during warm weather.
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<br>I expect short barrel life, but that's okay, since this will be used on long-range predators, large varmints, and deer-sized game.
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<br>If I have made any grossly inaccurate statements or assumptions, please set me straight.
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<br>George

GeorgeS,
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<br>I believe you have the powder burn rate exactly backwards.The faster powder would make it even more overbore.
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<br>Sounds like a great project. What is your barrel length?
<br>I believe you would be better off by using a very slow burning powder that fills up the case and still produces low to moderate pressure. Good luck

The term Overbore was used by the dean of wildcat cartridges quite a bit. I think Ackley ment the law of diminishing returns. It seems with a cartridge like you propose ( and one that I made up in .224") that adding a lot more powder yeilds very little extra velocity.

Cheaha,
<br>You may be right. I originally typed 'slower powder' then changed it because I thought since you less fast powder to produce a given velocity, that would change the equation.
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<br>The barrel has finished up at 27". It is a Shilen Match Grade C-M #4 contour. The action is a Remington 700 that will be squared and the bolt lugs lapped.
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<br>The factory stock is the nicest I've ever seen on a Remington, so I'm going to try to use that; I may add an H-S Precision or McMillan stock later if warranted.
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<br>I've already got a .300Wby, and three 7mmSTWs, so this will be for long-range on predators and lighter game. Obviously, pelt damage is not a consideration.
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<br>Don,
<br>I would think larger charges of slow powder would cause you to run into the law of diminishing returns more than smaller charges of fast powder (where you have to worry about excessive pressures).
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<br>George
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In my opinion,until you reach a true situation of "diminishing returns", there is no true overbore. If the shooter is willing to accept the longer barrel lengths, greater amounts of powder, recoil and muzzleblast, the the term "overbore" is relative. Shoot what makes you happy. I happen to like the .257 STW, the 7mm Ultra Mag, the .30-.378 and their kin. Are they efficient? Heck no. Are they necessary? Heck no. Are they fun? Heck yes.
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<br>2nd

By definition, a cartridge is overbore when a maximum load of the slowest available powder does not fill the case 100%. HTH, Dutch.

Well, I see that while I was gone, Dutch gave you the right slant -- as far as he went. Let's go further.
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<br>First, let's get the words right. "Overbore" isn't the word. The basic term is BORE CAPACITY.
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<br>The basic term is TWO words, both critical to meaning and understanding. A cartridge is UNDER bore capacity, right AT bore capacity, or OVER bore capacity. "Overbore" is a meaningless nonword in this application.
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<br>A cartridge is at or under bore capacity as long as there's a canister powder reasonably available that's SLOW enough to fill the case WITHOUT raising peak pressures above the safe maximum for the firearm that fires that cartridge.
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<br>Most cartridges are well under bore capacity since the introduction of powders slower than IMR 4350. Many that used to be over bore capacity are now UNDER bore capacity, since the newer, slower powders (4831 through 50BMG) make it not only possible but easy to fill the case (hence the word "capacity") without the resistance (hence the word "bore") to the propelling expansion of that powder's gas being great enough to raise the peak pressure of the expanding gas to a dangerous level.
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<br>IOW, many cartridges that were OVER bore capacity in Ackley's day (because the slowest canister powders available then were 4350 and the just-introduced surplus 4831) are no longer over bore capacity if a caseful (100% of the net capacity under the bullet) of, say, IMR-7828 doesn't produce dangerously high peak pressures in that cartridge, with that bullet.
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<br>Today, a cartridge is OVER bore capacity if a caseful of the slowest available canister powder is excessive (produces dangerously high peak pressures) for the firearm that fires that cartridge. I don't know of any. I haven't explored the slowest, which is probably Hodgdon's 50MG. My guess is that a .30-06 necked to .17 would be over bore capacity even for .50 machine-gun powder, and I'm sure that a .17-.50 MG would be 'WAY over bore capacity.

" .17-.50 MG would be 'WAY over bore capacity."
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<br>ROTFLMAO! Over bore indeed. Love your sense of humor Ken, Glad you are back.
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Thanks, Ken.
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<br>I'll go with IMR7828, Reloder 22, Reloder 25, and Retumbo in the .257STW.
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<br>George

I don't think it's what Dutch said. I think Ackley ment that you don't get much more velocity after a certain point. There was plenty of 4831 around in Ackleys day, maybe even more than today!
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<br>Ackley was quick to label such cartridges and it is a general term so that was his opinion. Others can have their opinon.
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<br>Taking .30 caliber as an example overbore comes somewhere near the .300 Win Mag or .300 Weatherby. Now this is my opinion.
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Every day, these forums prove that thinking isn't an absolute requisite for an opinion.

These forums also prove that bias is almost everywhere.
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<br>Just mention the name Ackley here an it gets hot!
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<br>Parker Ackley was the father of modern wildcating. As such what Ackley said still goes. In my view, subject to debate, is that by "overbore" Ackley ment "over bore capacity" for efficiency.
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Don, absolute nonsense. Parker Ackley popularized wildcatting by standarding body taper and shoulder angles. The concept of limited taper had been proven by men such as Donaldson decades before then. Moreover, such men as Page, Howe and Whelen achieved the same ballistic results, without having to fireform cases into shapes that result in operational (feeding) problems. Don't believe me? Compare Ackleys beloved 25 Roberts improved against the 25 Souper Pooper by Page.
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<br>Even such men as the German Brenneke were abreast of Ackley, without having someone to follow, 75 years earlier.
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<br>If Brenneke or even Newton had had Ackley's gift of gab, the civilian firearms world would have evolved much more rapidly. JMO, Dutch.

Dutch,
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<br>Absolute nonsense. Kilbourn, Donaldson, Woytkins, lovell, Risley, Niedner, Landis, Gebby, Whelan, Newton and others were pre war wildcatters. They pursued accuracy and performance. It's my view that Ackley came along at a time and published books and articles just at the time when B&A ( Bheiler and Astles) discovered the sizing up principle of bullet making. All the shoulder angle stuff done before had little to do with results. It was the bullets all along. Then about 1948 Sierra made production bullets by that principle and the problems were over.
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<br>So while the others did just as much or more work their results were stillborn and Ackley is the father.
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Duplicate post edited.
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Yes, Don, there was plenty of 4831 available during P O's time. I was handloading in those days myself, and I remember when the late Bruce Hodgdon was first making 4831 available in handloader canisters. P O's days bracketed the introduction of 4831 to handloaders. So I knew whereof I spoke in my earlier post, and what I wrote is the truth of the facts, whatever my or your opinion of it may be.
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<br>Whatever your opinion may be, the interior ballistics of bore capacity, bore-capacity cartridges, etc, are facts as I have written. Regardless of what you and I think of P O Ackley or his work, he did not define or explain the term, as far as I know.
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<br>Unless you're an experienced and careful lexicographer using citations from a multitude of sources, defining a term by its use in context is always risky at best (especially when you're casually using only one person's use of it as your context). For example -- I have my writing students jot down what they think the word "prodigal" means, and their definitions always reflect the context of the New Testament parable of the prodigal son -- "traveler," "wanderer," etc. The word means "wasteful." And "bore capacity," which P O clearly understood and often referred to but did not invent and never defined or explained, is what I've said it is.
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<br>And for the record -- it isn't true because Ken Howell says it -- Ken Howell says it because it's true.

Ken,
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<br>Of course to me the meaning of "overbore" was defined by the way Ackley used it. I recall that he used it in a derogitory fashion and he used it on a lot of cartridges. It seems to me that he was not that pleased with a lot of the designs that were commissioned.
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<br>At any rate the term makes sense the way Ackley used it. A primary goal in cartridge selection by sportsmen is more and more velocity. There is a practical limit to that until radically new powder is developed or we go to sabots or some other device.
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<br>Regards
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<br>Don

<br> While we are at it there is another phrase loved by the "old timers". "Balance Point". This I always assumed to mean the point of where increased loading resulted is less increase in velocity. I doubt that a "point " exists.
<br>Good Luck!

Looking through Ackleys books, I noticed several loads using H-870, H-570, 5010 and "Machine Gun Powder".
<br>Weren't at least the Hodgdon powders considered readily available cannister powders?
<br>Ackleys books came out about 10 years before I started reloading, so I don't know what actually was "readily avasilable" in that era.
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