Before we all take this too seriously, one definition of overbore is someone who is OVERbearing and a big BORE. Some politicians come to mind.
More seriously, when talking about our favorite hobby, what kind of item are we talking about when we say, "overbore". If I read Ken correctly, the thing we are talking about is a cartridge. The SAAMI definition definitely talks about a firearm. Now both of those are respectable sources, but they are at odds with each other. Since it's not a religious/ethical issue, we can choose whatever definition best serves us. But which? And why?
Second, what do you have to know about the thing we are talking about, in order to declare it "overbore"?
I'm convinced that the 264 Mag is overbore, so I'm pretty sure there is such a thing as overbore.
If the definition is that you cannot safely fire a cartridge filled with the slowest powder you can find, then there is no such thing as overbore, because I can fill any cartridge with an uncommonly slow powder, and maintain a safe pressure. That would mean that there is no such thing as overbore, and I don't think that is correct.
If the definition is that there is no powder available that will allow relatively efficient conversion of the chemical energy in the powder to kinetic energy of the bullet, then I think there is a workable definition. The bore is too small, and/or too short to allow an efficient conversion. A rifle is the original thermodynamic engine, and the same principles govern as those that govern a gasoline engine. The efficiency of any theromodynamic engine cannot exceed the energy extracted, divided by the total energy available. Some bores and strokes are more efficient than others. (And that has almost nothing to do with EFFECTIVENESS.)
So, what do you think? Is it the cartridge? Or the firearm? And what test do you perform, on what variables, to declare something overbore?
� When you use "overbore," you're using a nonword substitute for the old (real) term over bore capacity. I saw this term used at its beginning and learned and understood then what it meant � lore that most of the older men who knew are no longer alive (or otherwise silent) to explain, and younger shooters haven't known in the first place. I saw it evolve etymologically into a nonword that (a) descibes nothing and (b) refers obliquely to a relationship that was real enough then with the fewer powders that we had but no longer exists with today's several much-slower powders. When I tell you what the original term meant and why it no longer applies in the ultimate sense, I'm speaking from memory and long knowledge, not from either a senile ol' phardt's lunacy or some weird private opinion.
� So what does SAAMI say that it is? My copy of SAAMI's 1978 Glossary of Industry Terms does not list either overbore or bore capacity. Any later edition of this glossary that defines "overbore" as a real word includes a definition that some young guy wrote in total ignorance of the real, original term and its evolution into the nonword.
� A cartridge is over bore capacity altogether if there's no powder that's slow enough to fill its case without producing excessive pressures. With powders as slow as Hodgdon's 50-BMG, there probably no such thing as a cartridge that's over bore capacity. (Such a cartridge would have a very large case behind a very small bore. A .50 Browning necked-down to a .17 bore would probably be over bore capacity. But I don't think that anyone has built one of those yet.)
� A cartridge is over bore capacity for a specific powder if a caseful of that powder behind a specific bullet would produce excessive pressures.
� A cartridge is over bore capacity for a specific powder if a caseful of that powder is too fast (would produce excessive pressures) with any bullet.
So get over "overbore." It isn't a word, despite its wide common use, and it means nothing but unnecessary confusion. Instead, get used to using the term (and concept) bore capacity, and you'll find things making more sense to you in terms of (a) case capacities relative to (b) bore sizes (constrictions) that restrict free release of the chemical energy stored in the powder.
I'm no authority on the interior ballistics of the .264 Magnum, but my old gut assures me that it is indeed over bore capacity for a range of powders beginning with Bullseye for sure and possibly reaching as far as IMR-3031. But I doubt that it's over bore capacity for a range of powders possibly as quick as IMR-7828 and surely as slow as 50-BMG.
I'll have to see what QuickLOAD suggests about casefuls of some of the slowest powders and the .264 Magnum.
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"Good enough" isn't.
Always take your responsibilities seriously but never yourself.
Just because SAAMI says it, doesn't make it right. But here is what the do say. They clearly make the distinction between "overbore" and "over bore capacity", as you just did. Beyond that, they clearly refer to a firearm, and, in a muddled way, seem to be talking about efficiency.
Quote
OVER-BORE A shotgun whose barrel bore diameter is greater than the SAAMI maximum for that gauge.
OVER-BORE CAPACITY A firearm chambered for a cartridge which contains more powder than can normally be burned in that bore diameter and volume.
Their definition is quite problematic, since, taken literally, it means you have unburned powder exiting the muzzle. In almost all practical rifle situations, muzzle flash is incadescent, hot, deoxygenated gas, rather than burning powder.
Ackely's definition relates to a firearm, and very clearly uses efficiency as the basis.
If you use the efficiency definition, the firearm is over bore capacity, because at 25", the pressure is still at 20,000 PSI, so you have a lot of waste energy being vented when the bullet exits, and the chemical/kinetic energy conversion cannot be very efficient. (Measly 2900 fps)
Of course, if you lengthen the barrel, efficiency will improve. Does that mean that barrel length enters into the determination? That sort of offends my sensibilities.
If we use the safe caseful definition, then the cartridge is not over bore capacity with that powder.
Two different outcomes, from two different definitions....
I definitely was not around, or involved with the authors quoted by our esteemed Mr. Howell. I find his history of this term very enlightening.
But is not a word defined by the meaning conveyed with it's usage? Do definitions not change over time, whether we like it or not?
My understanding of the term "overbore" in the more modern usage is that of an adjective, of subjective value. Much as blue is "darker" than pink, and pink is "darker" than white.
With the modern usage, it would be correct to say that the 30-30 is "overbore" compared to the 30 carbine, or the 308 is "overbore" compared to the 30-30.
Is this usage incorrect if it is commonly understood between peers?
People who choose to brew up their own storms bitch loudest about the rain.
I think you are right, that the word means whatever we decide it means. The choice is a matter of convenience, and use. Some definitions are more useful than others. If we choose a definition that leads to contradictions, then it's not the most useful definition.
I'm really perplexed. Is it the cartridge that is over bore capacity? Or is it the firearm? Is it efficiency, as some authorities hold, or a safe caseful, as other excellent sources hold?
It would be helpful if there were one really clear definition, that didn't raise contradictions, that we all agreed on. (Figure the odds...) <img src="/ubbthreads/images/graemlins/frown.gif" alt="" />
I hope I am not straying too far off the topic here.............
To me, "overboreness" is important mainly in terms of helping predict barrel life.
I personally don't care how relatively efficient (high expansion ratio) or inefficient (low expansion ratio or "overbore") a particular chambering is. I do know that in general, a large capacity case firing a small diameter bullet will erode the throat faster than a small capacity case firing large diameter bullet. So for me, "overbore" has come to mean large capacity cases shooting small diameter bullets in barrels that have "shorter" life.
Geoff Kolbe took a stab a quantifying this. He suggested that the maximum efficient case capacity was bore diameter squared multiplied 1000 (BD^2 * 1000). Do the math and we find a case capacity of about 95 grains for the .308 bore. This is roughly halfway between a 300 Win and a 300 Wby. I think we could make a good "case" (pun intended) that 30 caliber cases of larger capacity than the 300 Win/Wby provide only marginal increases in velocity, though they burn significantly more powder and have much shorter barrel life.
To throw out some numbers, a 300 Win competition rifle is expected to be good for 1500 rounds, where Celt has reported 300 RUMs shooting out in 600 rounds. I understand there are always exceptions, and realize all these numbers are approximate.
Whether Kolbe is right or wrong with his suggestion of what the max efficient case capacity is, he did provide us with a constant, and a way to compare one chambering to another in terms of relative "overborness". The question I wanted to answer was would a 6mm Rem have as good of barrel life as a 22-250? I created a spreadsheet that makes this comparison. The case capacity figures come from the Barnes #2 reloading Manual, and are approximations only. I do not vouch for their accuracy. Variations in internal capacities of different brands of brass--even different lots the same brand, as well as bullet seating depth, must be considered to get precise numbers. The only way to know the capacity of a certain case in grains of water is to fill it and weigh it yourself.
Anyway, using Kolbe's formula, I was able to see that the 6mm Rem is only slightly more overbore than the 22-250. This lead me to believe the 6mm Rem would have approximately the same barrel life--all things being equal--as a 22-250. Then I eroded the throat of my 6mm Rem three-tenths of an inch when I put 85ish rounds through it in a 1000 yd BR match (barrel had around 250 total rounds), so that blew my theory out of the water. Then again, the Heavy Gun portion of an NBRSA 1000 yd match puts a LOT of heat in a barrel, especially a #5 contour............
That failure to validate my idea aside, I think if you look at the spreadsheet--which I don't know how to convert and attach--you'll see that in general, the chamberings that have a lower percentage of being overbore give better barrel life than the ones with a higher number. IE, the 308 Win has excellent barrel life and has a overborness figure of 56%, where the 300 RUM--which has relatively short barrel life--has an overboreness figure of 118%. BTW, the 264 Win comes in at 117%. Not sure this means anything to anoyne but me��� <img src="/ubbthreads/images/graemlins/crazy.gif" alt="" />
I have no problem at all with someone who shoots a rifle that will burn out in 1,000 rounds. That's an economic decision that some people have a reason to make.
Personally, I'd like to shoot as much as I can, as cheaply as I can. That's my priority. I like my 6.5x55, and I'm building a 7x57... definitely not everyone's cups of tea.
"Over bore capacity" is not evil. It's a matter of knowing your alternatives, and making the choice that is best for you. Bigger case, smaller bore = less barrel life, and more finicky loads. The key is being able to estimate that, before it happens.
